the movement of the progress bar may be uneven because questions can be worth more or less (including zero)…

Brief Explanations:

To determine the correct punctuation, we analyze the introductory phrase "It may sound crazy" and the conjunction "but" connecting two independent clauses (or a main clause). When an introductory phrase is followed by a conjunction introducing a main clause, we use a comma after the introductory phrase. The phrase "It may sound crazy" is an introductory element, so we place a comma after it, and "but" (a coordinating conjunction) connects the two parts. The correct structure is "It may sound crazy, but fatty fish is one of the healthiest things you can eat." The first option incorrectly adds a comma before "you can eat" (creating a comma splice), the second option incorrectly adds a comma after "but" (unnecessary), and the fourth option places the comma incorrectly after "but" instead of after the introductory phrase.

Answer:

C. It may sound crazy, but fatty fish is one of the healthiest things you can eat.