This is a typical question, the correct answer is A, and the analysis is as follows:

Cook is a stern but kind captain. He often takes care of the sailors on the ship.

Pronoun one, as an appositive, refers to Captain Cook; Who is used to guide the non-restrictive attributive clause to modify one, further explaining how kind Captain Cook is. The comma in the sentence is used to emphasize "universality", indicating that it is a regular thing to take care of sailors.

Cook is a strict but good captain, and he will naturally take care of the sailors on his ship. Further simplification can be: Cook is the captain, and he takes care of the sailors.

The link is the source of this question and the reference answer (Question 29) for your reference.

I wish you progress in your study!