The first political passengers on modern railroad cars were a president, a senator, and a governor that had been traveling to Washington, D.C. together on the Liberty Express in 1907.
A. The first political passengers on modern railroad cars were a president, a senator, and a governor that had been traveling - Verb - " That had been " - Wrong
B. The first political passengers on modern railroad cars were a president, a senator, and a governor, who traveled-Correct
C. A president, a senator, and a governor were the first political passengers on modern railroad cars who traveled -
who refers to nearest noun ( Mordern rail cars) - Wrong
D. A president, a senator, and a governor who were the first political passengers on modern railroad cars to travel - Relative Pronoun " who were the first political leader " - refers to only the Governor ... " SO IT IS WRONG "
E. A president, a senator, and a governor were the first political passengers on modern railroad cars traveling - In this sentence Main verb should be " traveling " - it just came here as a participle. So wrong