If the milkman . . . . . . . . tell him to leave two liters.

The correct answer is: A. comes

The sentence is in the present tense, so the verb “to come” should also be in the present tense. The other options are in the past tense (B), present perfect tense (C), and future tense (D).

Here is a more detailed explanation of each option:

• A. comes is the present tense form of the verb “to come.” It is used to describe an action that is happening or will happen in the near future. In this case, the milkman is expected to come to the house soon, so the present tense is the most appropriate tense to use.
• B. came is the past tense form of the verb “to come.” It is used to describe an action that happened in the past. In this case, the milkman has already come to the house,
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  so the past tense is not the correct tense to use.
• C. has come is the present perfect tense form of the verb “to come.” It is used to describe an action that happened at an unspecified time in the past and has a result that continues into the present. In this case, the milkman has come to the house at some point in the past and has left two liters of milk, so the present perfect tense is not the correct tense to use.
• D. come is the infinitive form of the verb “to come.” It is used to describe an action that is possible or desired. In this case, the milkman is not necessarily expected to come to the house, so the infinitive form is not the correct tense to use.