The correct option is:

B. was

The rule at play here is subject-verb agreement. When the subject of a sentence is composed of two or more nouns or pronouns connected by "neither...nor," the verb should agree with the noun or pronoun closer to it.

In this sentence, the subject is "neither the players nor the trainer." Both "players" and "trainer" are nouns, but they are connected by "nor," indicating a negative correlation between them. According to the rule, we choose the verb form that agrees with the noun or pronoun closest to it. 

In this case, "trainer" is closer to the verb, so we use the singular form of the verb, which is "was." Therefore, the correct sentence is: “Neither the players nor the trainer was in the room.”