The correct option is A. A city or vacation spot located near the ocean is in danger of a tidal wave.
The subject of the sentence is compound. That means that it has two noun phrases that are joined. In this case, the two noun phrases are joined by "or" which gives a sense of choice between one thing or the other. Moreover, each noun phrase in the subject are in singular form. As a result, the verb should be in its singular form as well.
