The correct personal pronoun to complete the sentence is the subject pronoun, C. I: "The funniest performers were Karl, Mike, and I.
The compound object "Karl, Mike, and I" is a subject complement.
A subject complement is a noun, a noun phrase, a pronoun, or an adjective that follows a linking verb to restate the subject of the sentence.
A linking verb acts as an equal sign, the subject is or becomes the object.
> performers = Karl, Mike, and I
A noun or a pronoun subject complement is called a predicate nominative.
A pronoun that functions as a subject complement (a predicate nominative) is always the subjective form.