Which force keeps and object moving in a circle?

Centripetal force

Explanation:

Centripetal force acts on an object moving in a circle.  It points towards the center of the circle and has a magnitude equal to m v² / r, where m is the object's mass, v is the object's velocity, and r is the radius of curvature.

initial speed when it is dropped is 150 ft/s

now at the same moment the acceleration due to gravity is perpendicular to its velocity

so as we know that

now we have centripetal acceleration a = 32 ft/s^2 as its perpendicular to velocity

while tangential acceleration at the same moment will be zero

so

now radius of curvature is given as

so the radius of curvature just after it is dropped is 703.125 ft

distance = 150m

displ = root (50 ^2 + 100^2)

root(2500+10000)

root 125000