I don't know what was ___ on the playgroundA.happen B happened C chappens D happening

I don't know what was ___ on the playgroundA.happen B happened C chappens D happening
I don't know what was ___ on the playground
A.happen B happened C chappens D happening

I don't know what was ___ on the playgroundA.happen B happened C chappens D happening
B happened
我不知道操场上(刚才)发生了什么事
要被动语态,因为是什么被发生了,如果D的话,你不能有 was.
be+V 过去分词为被动语态!

d(happen 不能用被动语态)

D happen无被动

选D，happen不用作被动语态，又前面有be动词was，所以用进行时态。
句意为：我不知道操场上发生了什么。