In the absence of the chief guest, the host himself acted as his surrogate and presided over the function.

The correct answer is D. substitute.

A surrogate is a person who takes the place of another, especially in a legal or official capacity. In the context of the question, the host acted as a substitute for the chief guest, presiding over the function in his absence.

The other options are incorrect. A counterpart is a person or thing that corresponds to or is equivalent to another. An alliance is a formal agreement between two or more parties to cooperate or act together. An assistant is a person who helps another person with their work.

Here are some examples of how the word “surrogate” can be used in a sentence:

• The surrogate mother gave birth to a healthy baby boy.
• The judge appointed a surrogate to represent the defendant in court.
• The company hired a surrogate to fill in for the CEO while he was on vacation.