Answer:
D. They let me go.
Step-by-step explanation:
Go would be the most logical answer here.
One way that could help you is by putting it into a sentence and seeing which one sounds the best:
They let me...go
They let me...to go
They let me...going
Out of those three, the first one definitely sounds the most reasonable.
I hope my answer helped you! If you need more information or help, comment down below and I will be sure to respond if I am online. Have a wonderful rest of your day!