Question

Identify the use of the italicized noun clause.

The game will be won by whoever plays best.
subject complement
subject
object of preposition

Answer 1

Since you did not indicate the noun clause that is italicized, i'll proceed to identifying the noun clause in the sentence. If it is 'whoever plays best', then the function of the noun clause would be an object of preposition (preposition would be: 'by'). 

Answer 2

The right option is; object of preposition

A noun clause is a dependent clause that functions as a noun in a sentence. In sentences, noun clauses start with words such as whatever, how, whoever, when, where, and why. Also, noun clauses can function as subjects, objects of a preposition, direct or indirect objects, or predicate nominatives. In the given question, the noun clause in the sentence is; whoever plays best, and it is used as an object of the preposition; by.