Which one is a suitable explanation? None of them? If so, please suggest a better one.

Hi teachers,
According to this sentence, 'Tim has to work harder than the excavator operators'.

Which one is a suitable explanation? None of them? If so, please suggest a better one.
a) 'Harder is a comparative adverb because it compares the way Tim works compared with the excavator operators.
b) 'Harder' is a comparative adverb because it compares how Tim works compared with the excavator operators.
c) 'Harder' is a comparative adverb because it compares how Tim and the excavator operators work.

Thanks in advance.