Given the task definition and input, reply with output. In this task, you're given a question, a context passage, and four options which are terms from the passage. After reading a passage, you will get a brief understanding of the terms. Your job is to determine by searching and reading further information of which term you can answer the question. Indicate your choice as 'a', 'b', 'c', or 'd'. If you think more than one option is plausible, choose the more probable option to help you answer the question.

Question: How long had The Observer been operating as a going concern the year Harrisson started working there as a radio critic? Passage:During the Second World War Harrisson continued directing Mass-Observation and was radio critic for The Observer from May 1942 until June 1944. For much of this time he was in the army and gave up reviewing on leaving the UK. After service in the ranks he was commissioned as a second lieutenant in the Reconnaissance Corps on 21 November 1943. He had been recruited (some sources say by a confusion of names, despite his apparent suitability) for a plan to use the native peoples of Borneo against the Japanese. He was attached to Z Special Unit (also known as Z Force), part of the Services Reconnaissance Department (SRD: a branch of the combined Allied Intelligence Bureau in the South West Pacific theatre). On 25 March 1945, he was parachuted with seven Z Force operatives from a Consolidated Liberator onto a high plateau occupied by the Kelabit people. An autobiographical account of this operation (SEMUT I, one of four SEMUT operations in the area) is given in World Within (Cresset Press, 1959); there are also reports not always flattering from some of his comrades. His efforts to rescue stranded American airmen shot down over Borneo are a central part of "The Airmen and the Headhunters", an episode of the PBS television series Secrets of the Dead. The recommendation for his Distinguished Service Order which was gazetted on 6 March 1947 (and dated 2 November 1946) describes how from his insertion until 15 August 1945 the forces under his command protected the flank of Allied advances, and caused severe disruption to Japanese operations.
Links: a. Z Special Unit b. The Observer c. Kelabit people d. Second lieutenant
b