Answer: C. go
Step-by-step explanation:
We can tell this is past voice due to the word “did”
We can then eliminate ‘going’ because that is future continuous
Now going through the rest of the options and reading them with the sentence-
What they did was ‘gone’ to the police? Isn’t proper English.
What they did was went to the police?
(Can still be said but isn’t proper according to ‘rules’)
Which leaves us with c. -what they did was go to the police?
So therefore the answer should be c.