Answer:
C: on Friday, he worked the longest of all.
Step-by-step explanation:
The correct sentence that uses the superlative form of the adverb long is option C: "On Friday, he worked the longest of all."
The superlative form of the adverb long is "longest," and it is used to indicate that something or someone has the greatest length of time. In option A, the comparative form "more long" is used incorrectly. In option B, the comparative form "longer" is used instead of the superlative. In option D, the comparative form "longest" is used incorrectly, as it should be used to compare two things, not multiple things.
Hope that helps!