E should be the answer. This is a perfect example of a participial phrase which acts as an adjective to "served as a recruiting agent".
A) This is unappropriate as a conjunction (ie "and") would have been needed to link the two action "served" and "traveled"
B) double use of conjunction "and" is not right. Second action "traveled" should have been set off by a comma and then followed by conjunction "and" before the third action "exhorted"
C) This is a mixed up sentence. It wants to introduce a participial phrase with traveling but yet uses conjunction "and", which is not needed. This is also plagued by parallelism problem between "traveling" and "exhorted"
D) Parallelism problem between "traveling" and "exhorted"
E) Only correct sentence which introduce 2 participial phrases (with verbs ending in "ing"). Ask yourself the question: How
did Frederick Douglass served as a recruiting agent? Answer: By traveling
through the North and exhorting
Black men to join the Union army