'Six bananas' suggests that the answer must be plural. Also, it must be an action done to the bananas. Thus, it should fit into the sentence.
Option A is incorrect. 'Is crushed' is singular and thus, cannot be the answer.
Option B is correct. 'Are crushed' is plural. Also, it fits in the sentence. So, it is correct.
Option C is incorrect. 'should crush' suggests that the bananas are doing the action. Since that makes no sense, it is incorrect.
Option D is incorrect. 'Should be crush' is an incorrect phrase and so it cannot be the correct answer. The correct form is 'should be crushed'.