Take a look at the original sentence again:
Mark searched the nearly empty refrigerator for a midnight
snack his only choices were curdled milk,
stale pizza, and grape jelly, any combination of which he couldn't stomach.
You wanted to fix it this way:
Mark searched the nearly empty refrigerator for a midnight
snack, his only choices were curdled milk,
stale pizza, and grape jelly, any combination of which he couldn't stomach.
To add a comma between snack and his would cause an equally bad problem, a comma splice. A comma splice occurs when you have two main clauses joined with a comma alone. Mark searched the nearly empty refrigerator for a midnight snack is the first main clause. His only choices were curdled milk, stale pizza, and grape jelly is the second main clause. The spot between snack and his needs a stronger break than a wimpy comma.
You might want to consult the rules for fixing comma splices and fused sentences.
Go back to the sentence to try again.