Confusing incorrect question in my grammar book

The workers _________ $1,000 to plant the trees in the garden.

A) paid
B) to pay
C) paying
D) were paid

Can you tell me which answer is right and explain why?

If here is not the right place for this question, please forward me to a forum / community where I can get such questions answered.

You need to use the passive voice:

D) The workers were paid $1,000 to plant the trees in the garden.

In English, the indirect object is usually more idiomatic than the direct object as the subject in the passive voice of ditransitive verbs. Therefore, instead of $1,000 were paid to the workers to plant the trees in the garden, it is more idiomatic to use sentence (D) above.