TASK DEFINITION: You will be given a trivia clue, and the category it belongs to. You should answer with the best answer that belongs in the category and is described by the clue. For simplicity, answers should be in all lower cased letters.
PROBLEM: Category: CLASSICAL MUSIC 
Clue: Mendelssohn's "Songs without Words" were once the most popular pieces for this instrument

SOLUTION: piano

PROBLEM: Category: GET AN EDUCATION! 
Clue: God willing, you'll learn enough about religion to know this word defined as belief in a single god

SOLUTION: monotheism

PROBLEM: Category: TRAVEL & TOURISM 
Clue: One of the world's largest mosques, the Great Hassan II Mosque stands in this largest Moroccan city

SOLUTION:
casablanca