I _about the school trip before you reminded me.
left
was looking
had gone
were tidying
had forgotten
had left
went

Ответ:

I had forgotten

Объяснение:

I had forgotten about the school trip before you reminded me.

Я не буду расписывать, почему каждый не подходит,но здесь в любом случае past perfect так как дейтсвие было совершено до действия в прошедшем.. Таких 3:

had gone

had forgotten

had left

Ну и логично, что 1 и 3 здесь не подходят)