Your answer will be C, post-production.
Step-by-step explanation:
Pre-production is before the film is well, filmed, so you wouldn't be able to add any special effects during that part.
Production is when the film is being made, so again, you can't add anything to it unless if you have the rest of the film to make sure it fits in with everything.
And distribution is when the movie is released, so you can't add effects at all then.
Post-production is after it is filmed and before it is released, and is when all the effects are added.