The ____________ charge in that restaurant is 25%. I think we should go somewhere cheaper!

The word "service" is the correct choice in this context because it refers to the act of serving customers in a restaurant, which often includes a mandatory charge added to the bill for the waitstaff's work. The phrase "service charge" is commonly used in the hospitality industry to denote an additional fee for the service provided, aligning perfectly with the context of discussing dining expenses.

The other options do not fit as they convey different meanings or grammatical functions. "Serve" is a verb, while "servant" refers to a person who serves, and "servicing" typically relates to maintaining or repairing something rather than providing customer service. Therefore, "service" is the appropriate term to convey the desired meaning in this sentence.