They were having a meeting in the lecture hall when the door

这题应该先B,burst.
forced:obtained or imposed by coercion or physical power,表示有外力将门推开,强调在外力下被推开.
burst:break open or apart suddenly and violently, especially as a result of an impact or internal pressure,表示门被突然而且很粗暴地打开,特别强调打开门后对内部有造成影响.这句话潜在意思是,他们在开会,在遭遇门被打开后,影响了会议的进行.
break:separate into pieces as a result of a blow, shock, or strain…这词只是表示在外力下断裂,明显不符合这句话的意思.
tore:pull (something) apart or to pieces with force…是tear的过去分词形式,表示用力撕碎,撕裂等意思