The PLO began to negotiate with Israel in 1991 during the Madrid Conference, indicating a major change in their previously strained relationship. In contrast, 1978 and 1987 did not involve formal negotiations between the two parties. Therefore, the correct answer is 1991. ;
The PLO began to negotiate with Israel in 1991 during the Madrid Conference, marking a significant change from previous hostility. Prior events like the Camp David Accords and the First Intifada did not involve direct negotiations with the PLO. Thus, the correct answer is 1991.
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