Answer:
C. go
If you plug each choice into the blank, through the process of elimination, it will be easier to choose the right one.
Are you looking forward to go to London?
Are you looking forward to going to London
By doing that, you can see which one makes more sense, The only way going would work, is if the "to" after forward is replaced with "on" like this:
Are you looking forward on going to London?
Hope this helped!
