To answer this question, we need to understand the correct form of the verb "être" (to be) to use with the subject "monsieur" (sir).

Option A) Tu es - This option is incorrect because "tu" is the second-person singular pronoun, which is used to address someone informally. However, when addressing someone formally, like "monsieur," we use the second-person plural pronoun "vous."

Option B) Ils sont - This option is incorrect because "ils" is the third-person plural pronoun, which is used to refer to a group of people. In this sentence, we are addressing only one person, so we should not use "ils."

Option C) On est - This option is incorrect because "on" is an informal way to say "we" or "one." When addressing someone formally, like "monsieur," we should use the second-person plural pronoun "vous."

Option D) Vous êtes - This option is correct because "vous" is the second-person plural pronoun, which is used to address someone formally. When addressing someone like "monsieur," we should use "vous" as a sign of respect.

The correct answer is D) Vous êtes. This option is correct because it uses the appropriate form of the verb "être" with the subject "monsieur."

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