Explanation
Correct answer: C.
Choice C is the best answer. The convention being tested is the use of verb forms within a sentence. Relative clauses, such as the one beginning with "how," require a finite (tensed) verb, a verb that can function as the main verb of a clause. This choice correctly supplies the clause with the finite past tense verb "served."
Why the other choices are wrong
Choice A
Choice A is incorrect because it results in an ungrammatical sentence. The nonfinite participle "serving" doesn’t supply the clause with a finite verb.
Choice B
Choice B is incorrect because it results in an ungrammatical sentence. The nonfinite participle "having served" doesn’t supply the clause with a finite verb.
Choice D
Choice D is incorrect because it results in an ungrammatical sentence. The nonfinite to-infinitive "to serve" doesn’t supply the clause with a finite verb.