Answer:
(a) the image is attached below
(b) v = 0.237 m/s
Step-by-step explanation:
(a) The free-body diagram can be observed in the attached image
(b) To find the maximum speed you take into account the centripetal force:

m: mass = 0.80kg
v: tangential speed of the stone = ?
r: radius of the circular trajectory of the stone = 0.90mm = 0.90*10^-3 m
The force cannot exceed 50.0N, then you have:

hence, the maximum speed the stone can attain without breaking the string is 0.237m/s