The diameter and depth of blast holes are generally kept constant depending on . . . . . . . . of equipment.

Answer: Option B

The diameter and depth of blast holes are generally kept constant depending on the size of equipment. In surface mining operations, the size of the equipment used for drilling blast holes determines the size of the blast holes that can be drilled. The diameter and depth of the blast holes must be sufficient to accommodate the explosive charge and ensure that the blast is effective in breaking up the rock mass.

Option A, Base, Option C, Length, and Option D, Highwall, are not directly related to the size of the equipment used for drilling blast holes.

Therefore, the answer to this question is Option B, Size.