Review the original item:
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 could not stomach.
You chose the correct way to fix the error:
Mark searched the nearly empty refrigerator for a midnight
snack,
but his only choices were curdled milk, stale pizza, and grape jelly,
any combination of which he could not stomach.
Mark searched the nearly empty refrigerator for a midnight snack is a main clause. His only choices were curdled milk, stale pizza, and grape jelly is the second main clause.
Using a comma and a coordinating conjunction like but is always a good option for fixing a fused sentence.