The correct answer is C: is. This question involves subject-verb agreement with correlative conjunctions, specifically "neither...nor." When "neither...nor" connects two subjects, the verb must agree with the subject that is closer to it. This is known as the rule of proximity.
In the sentence "Neither the players nor the coach __________ available," the two subjects are "the players" (plural) and "the coach" (singular). The subject closer to the blank (and thus the verb) is "the coach," which is a singular noun. Therefore, the verb must be singular to agree with "the coach." "Is" is the singular form of the verb "to be" in the present tense, making it the correct choice. If the closer subject had been plural, for example, "Neither the coach nor the players are available," then a plural verb would be required. This rule ensures proper grammatical agreement when dealing with compound subjects joined by "neither...nor" or "either...or."