You must dispense . . . . . . . . his service.

in
at
with
of

The correct answer is: of.

To dispense something means to give it out or distribute it. In this case, the speaker is telling the listener to give out or distribute the service that the listener has received from the person mentioned. The preposition “of” is used to indicate that the service is being given out or distributed from the person mentioned.

The other options are incorrect because they do not make sense in the context of the sentence. The preposition “in” is used to indicate a location or position. The preposition “at” is used to indicate a time or place. The preposition “with” is used to indicate a tool or instrument.

Here are some examples of how the preposition “of” can be used in other sentences:

  • The teacher dispensed the test papers to the students.
  • The doctor dispensed the medicine to the patient.
  • The store dispensed the cash to the customer.