Choose the correct answer. When a traffic light turns red, it _ the motorist must stop.

The sentence requires a verb to complete the clause. 'Indicates' is the third-person singular present tense form of the verb 'to indicate', which fits grammatically and contextually. The other options are an adverb ('indicatively'), an adjective ('indicative'), and a noun ('indication'), none of which function correctly as the main verb in this sentence.

The correct word to complete the sentence is indicates.