Let us realize that if there were no comma before who, then the pronoun 'who' would directly refer to only the governor and not to the others. That means that only the governor traveled to Washington D. C, while the president and the senator were headed to someplace other than Washington D.C. Hence, in order to refer to all the three political heavyweights who traveled together to the ultimate destination, the comma before 'who' becomes necessary.
A relative pronoun can refer to an object of a preposition and it absurdly indicates that the cars traveled to Washington D.C.
E has the same problem as ice where the verb+ing modifier naturally is modifying the cars.
(A) The first political passengers on modern railroad cars were a president, a senator, and a governor that had been traveling
(B) The first political passengers on modern railroad cars were a president, a senator, and a governor, who traveled
(C) A president, a senator, and a governor were the first political passengers on modern railroad cars who traveled
(D) A president, a senator, and a governor who were the first political passengers on modern railroad cars to travel
(E) A president, a senator, and a governor were the first political passengers on modern railroad cars traveling
Some posters have wrongly transcribed choice A as with a 'who traveled" rather than as 'that traveled' as found in the prompt and have claimed A is the choice. It has to be 'that traveled' and hence A can never be the correct answer.
A) The first political passengers on modern railroad cars were a president, a senator, and a governor, who traveled
The answer is, therefore, B with 'who traveled' modifying the three political heavyweights.
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