Choose the word that best completes the sentence.

Karina and _____ are working together to build the model.
A. her
B. me
C. she
D. us

"Karina" is part of the subject, so the pronoun must agree in case.

Hence, C. she

She and Karina are working together.
You could also say

The correct answer is D. us

D. us

"Karina is working with her." But in the context of the given sentence, "Karina and she" is the correct subject, and "us" is the correct object, so the answer is still D. us.