You have to understand the conditions of the problem. Theta is measured with the vertical, not the horizontal. The givens are
Theta is measured with the vertical
The charges are equal (an assumption)
The masses are equal (Given)
Discussion A and D
The problem with A is that it is closely connected to D. When you increase the charge, the force will go up. To compensate for the increase in force, the distance will have to increase between the 2 charged bodies. Either A is true and D is true or they both are false. In this case, both are false. You yourself eliminated D. So A goes with it.
Discussion on C
Nothing remains the same once you increase the Charge. C cannot be the answer. When the charge is increased, the Force is unbalanced and to balance it, the distance will increase. So C will be wrong because everything changes.
B is the answer. If the only way to balance the force is to increase the distance, it will mean that theta increases as well.