The correct answer is A. Three times.
The pitch of the main bars in a simply supported slab is the distance between the centers of two adjacent main bars. The effective depth of a slab is the distance from the top of the slab to the centroid of the tension reinforcement.
The pitch of the main bars in a simply supported slab should not exceed its effective depth by more than three times. This is because the main bars are responsible for resisting the tensile stresses in the slab. If the pitch of the main bars is too large, the tensile stresses in the slab will be too high and the slab will be more likely to crack.
The other options are incorrect because they do not represent the correct relationship between the pitch of the main bars and the effective depth of the slab.