Select the word that makes this sentence grammatically correct.
The group knew it was _________ responsibility to turn in the assignment on time, but they still didn’t do it.

"Their" is an adjective, so it modifies a noun. In this sentence, it was "their" responsibility. "There" relates to a place or location. "They're" is a contraction of "they are". "The" is an article and would not make sense here.