Question

A block of weight 15N is hanging from a rigid support by a string. What force is exerted by:
a) Block on string
b) string on the block

Answer 1

the forces acting on the block are  the weight (due to gravitation) and the tension force in the string pulling it upwards.
  Since the block is stationary and in equilibrium,  the two forces are equal and opposite.  Hence, the tension force exerted by the string on the block = weight = 15 N.

 The forces on the string at the lower end are :  tension in the string and the weight of the block.  Thus the force exerted by the block on the string is equal to the weight of the block = 15 N.

Answer 2

your answer is
(a)= 15N downwards (weight)
(b)= 15N upwards (tension)