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

Answer: Option D

Countertpart : a person or thing that corresponds to or has the same function as another person or thing in a different place or situation.
Alliance : a union or association formed for mutual benefit, especially between countries or organizations.
Assistant : a person who ranks below a senior person.
Substitute : a person or thing acting or serving in place of another.
Surrogate : a substitute, especially a person deputizing for another in a specific role or office.

Synonym of Surrogate is Substitute