Question: Zulema Castro de Peña was a member of what organization that began to march in 1977 at the Plaza de Mayo in Buenos Aires?

Context paragraphs:
The Mothers of Plaza de Mayo: The Mothers of Plaza de Mayo (Spanish: "Las Madres de la Plaza de Mayo" ) is a 1985 Argentine documentary film directed by Susana Blaustein Muñoz and Lourdes Portillo about the Mothers of the Plaza de Mayo.  It was nominated for an Academy Award for Best Documentary Feature.

María Adela Gard de Antokoletz: María Adela Gard de Antokoletz (October 11, 1911 – July 23, 2002) was one of fourteen women who founded the Mothers of the Plaza de Mayo movement.  Her son Daniel was abducted in November 1976.  Later, when she was working for the provincial courts in Buenos Aires, she joined other mothers of missing children to found the Mothers of the Plaza de Mayo.  This group was dedicated to finding out what had happened to their missing children.  As part of this group, María Adela Gard de Antokoletz led protest marches every Thursday on Buenos Aires's Plaza de Mayo holding a picture of her son.  Throughout her life, she received death threats because of her work, but she refused to forget what had happened to her son.

Buenos Aires Metropolitan Cathedral: The Buenos Aires Metropolitan Cathedral (Spanish: Catedral Metropolitana de Buenos Aires ) is the main Catholic church in Buenos Aires, Argentina.  It is located in the city center, overlooking Plaza de Mayo, on the corner of San Martín and Rivadavia streets, in the San Nicolás neighbourhood.  It is the mother church of the Archdiocese of Buenos Aires and the primatial church of Argentina.

Avenida de Mayo: May Avenue (Spanish: Avenida de Mayo ) is an avenue in Buenos Aires, capital of Argentina.  It connects the Plaza de Mayo with Congressional Plaza, and extends 1.5 km in a west-east direction before merging into Rivadavia Avenue.

Adelina Dematti de Alaye: Adelina Ethel Dematti de Alaye (June 5, 1927 – May 24, 2016) was an Argentine human rights activist who co-founded the Mothers of the Plaza de Mayo, an organization of mothers whose children disappeared during the Dirty War of the 1970s and early 1980s.  She became known as "la madre fotógrafa," or "the mother photographer," for her documentation of the Argetine dictatorship and the Mothers of the Plaza de Mayo through her photography.  Her collection was later declared a "Memory of the World" by UNESCO.  In addition to her work with the Mothers of the Plaza de Mayo, Dematti was also active within the Permanent Assembly for Human Rights (APDH).

Mothers of the Plaza de Mayo: The Mothers of the Plaza de Mayo (Spanish: "Asociación Madres de Plaza de Mayo" ) is an association of Argentine mothers whose children "disappeared" during the state terrorism of the military dictatorship, between 1976 and 1983.  They organized while trying to learn what had happened to their children, and began to march in 1977 at the Plaza de Mayo in Buenos Aires, in front of the Casa Rosada presidential palace, in public defiance of the government's state terrorism intended to silence all opposition.

Zulema Castro de Peña: Zulema Castro de Peña (c. 1920 – January 22, 2013) was an Argentine human rights activist and member of the Mothers of the Plaza de Mayo.  Her two sons, Jesús Peña and Isidoro, disappeared during Dirty War of the 1970s, leading to her lifelong campaign for human rights and answers for the relatives of the "disappeared."

Line A (Buenos Aires Underground): Line A is the oldest line of the Buenos Aires Underground.  Opened to the public on 1 December 1913, the first underground line in South America, the Southern Hemisphere and the Spanish-speaking world, it made Buenos Aires the 13th city in the world to have an underground transport service.  The line stretches 9.8 km from Plaza de Mayo and San Pedrito and runs under all of the Avenida de Mayo and part of the Avenida Rivadavia, and is used by 307,000 people per day.

Veinticinco de Mayo, Buenos Aires: Veintecinco de Mayo (or 25 de Mayo) (in English: 25 May) is a town situated in the centre of Buenos Aires Province in Argentina and has a population of 22,581 (2001).  Founded on 8 November 1836, it is the capital city of the partido of the same name.  The city takes its name from 25 May 1810, the day on which Buenos Aires began the process leading to a formal declaration of independence by forming its own junta ("assembly") and inviting the other provinces to join it.  The city is 220 km from Buenos Aires, and 240 km from La Plata, the provincial capital.

Bombing of Plaza de Mayo: The Bombing of Plaza de Mayo was a massacre which took place in Buenos Aires, Argentina, on 16 June 1955.  On that day, 30 aircraft from the Argentine Navy and Air Force strafed and bombed Plaza de Mayo square in Buenos Aires, in what remains to this day the largest aerial bombing ever on the Argentine mainland.  The attack targeted the adjacent Casa Rosada, the official seat of government, as a large crowd was expressing support for president Juan Perón.  The strike took place during a day of official public demonstrations to condemn the burning of a national flag allegedly carried out by detractors of Perón during the recent procession of Corpus Christi.  The action was to be the first step in an eventually aborted coup d'état.  The number of identified bodies was put at 308 —including six children— plus an indeterminate number of victims that couldn´t be identified.

Supporting paragraphs: Zulema Castro de Peña Mothers of the Plaza de Mayo
Answer: Mothers of the Plaza de Mayo