The correct answer is option C.
In the given sentence we have the verbs "served" and "reformed" both conjugated in the Simple Past Tense.
In order to correctly complete the parallel structure we should use option C (led the rough riders up San Juan hill) where the verb "led" is also conjugated in Simple Past Tense.
The full sentence would be: "Before becoming president, Theodore Roosevelt served as deputy sheriff, reformed a police department, and led the rough riders up San Juan hill".
