ans is d

take any point mass let say a

b and c will exert gravitational force on a . they will be = in magnitude= f

let the angle bw the 2 forces on a be  .than the resultant of both the f's is

3f...

similarly for b and c also the resultant gravitational force is 3f.inclined at angle 30 to each side

now all these forces meet at the centre of the triangle. now the the radius of rotation is R =r/ 3

so we get

mv²3/r = Gm3/r²

so v =Gm/r

 

rate if useful