While I cannot see your answer choices, the correct answer would be one that has a coefficient that is a multiple of 3 and has an x raised to an exponent of no more than 2. It could have any other variables in it.
The reason this has to be is because of the GCF. Since 3 is part of the GCF, and the other monomial has a coefficient of 3, this means the missing monomial must be divisible by 3 as well.
Since x² is part of the GCF, the missing monomial must contain x². However, if it were to contain x³ or anything larger, the GCF would have x with a higher exponent.