ANS:B - 43%

The constant angle arch dam is a special type of variable radius arch dam in which the central angle of the horizontal arch rings is of the same magnitude at all elevations.

Arch-dam-type.

It has been shown that the volume of concrete is minimum when the central angle is 133' 34'.

A variable radius arch uses about 58% of the concrete required in constant radius arch dams. A constant angle arch dam requires about 42.6% of the concrete required in constant radius arch dams.

Hence a constant angle arch dam is the most economical.