The correct answer is (a) did not
The sentence reads: "…if they did not submit their assignments by the end of the day, they might get an opportunity to watch the match."
The sentence uses a conditional structure ("if…might"). The motivating contrast requires a negative condition — failing to submit leads to the reward. "Did not" fits grammatically (past tense in the if-clause) and logically. The other options are grammatically or contextually incorrect here.