The answer is "B": In the centre ring were four monkeys on bicycles. It cannot be "A" because the verb used there is in the singular form when it should actually be in the plural form of "was". It cannot "C" because the verb used there is in the plural form when it should be in the singular form of "is". And, it cannot be "D" because the verb used there is in the singular form when it should actually be in the plural form of "were".
Therefore, it is "B" as the verb here actually agrees with its subject.
