What is the simple subject in the following sentence taylor, kevin, and brenda have been working hard on their project for the science fair

The subject of a sentence is the person or thing that is doing the action. Often it comes first in a sentence. A simple subject is only the person or thing. A complete subject is the person or thing that is doing the action plus any modifiers (a, an, the, and other adjectives for example)
EXAMPLE: The fuzzy dog ate his dinner.
The simple subject is: dog
The complete subject is: The fuzzy dog. (The and dog describe/modify dog.)

The simple subject in the question is Taylor, Kevin, and Brenda.
Since there are three parts to the subject (each of the three people) it is also considered to be a compound subject.

Omit means cleave out .