At the end of the day, John is the ________

of all workers.

A. tired
B. tireder
C. tiredest
D. most tired

I think it's choice C .

Choice c follows the rule. However, "tired" seems to be an exception. Most English-speakers would say "most tired."

http://thegrammarexchange.infopop.cc/eve/forums/a/tpc/f/340600179/m/5256000434

To determine the correct answer, we need to compare the degree of tiredness between John and all other workers. In this case, we are looking for the superlative form, which implies the highest degree of comparison.

The options provided are "tired," "tireder," "tiredest," and "most tired."

A. "Tired" is in the positive form.
B. "Tireder" is in the comparative form, comparing John's tiredness to some other worker but not to all workers.
C. "Tiredest" sounds like the superlative form, but technically, "tireder" is already in the comparative form, so we don't need to add the "-est" suffix.
D. "Most tired" is the correct superlative form, comparing John's tiredness to that of all other workers.

Therefore, the correct answer is D, "most tired."