What word is best to substitute for the underlined words in the following sentence? The boy watched the lights in the house go off.

In the sentence, 'The boy' is the subject performing the action of watching the lights go off. Therefore, the correct substitute should be a subject pronoun. 'He' is the appropriate pronoun to replace 'The boy.' 'Him' and 'his' are object pronouns and possessive adjectives, respectively, and would not be suitable for this context. 'They' is a plural pronoun and does not match the singular subject 'The boy.' Thus, 'he' is the correct word to use in place of 'The boy' in this sentence.