To understand why the correct answer is option 'A' (make the vertical sum of the forces equal to zero), let's first understand what the normal force is and how it relates to the supports of an object.

The normal force is the force exerted by a surface to support the weight of an object resting on it. It acts perpendicular to the surface and prevents the object from falling through the surface. In the case of supports, the normal force is the force exerted by the supports to balance the weight of the object.

Now, let's break down the options given:

a) Make the vertical sum of the forces equal to zero:
- The vertical sum of forces refers to the sum of all the forces acting in the vertical direction (upwards and downwards).
- By making the vertical sum of forces equal to zero, we are essentially ensuring that the net force in the vertical direction is zero.
- This is important because when an object is in equilibrium (i.e., not accelerating), the net force in all directions must be zero.
- In the case of supports, the vertical sum of forces being equal to zero ensures that the object is not moving vertically and is balanced on the supports.

b) Make the horizontal sum of the forces equal to zero:
- The horizontal sum of forces refers to the sum of all the forces acting in the horizontal direction (leftwards and rightwards).
- While the horizontal sum of forces is important for determining the acceleration or motion in the horizontal direction, it does not directly relate to the normal force on the supports.

c) Make the moment sum of the forces equal to zero:
- The moment sum of forces refers to the sum of all the moments (torques) acting on an object.
- While the moment sum of forces is important for rotational equilibrium, it does not directly relate to the normal force on the supports.

d) Make the rotational sum of the forces equal to zero:
- This option is similar to option 'c' and refers to the sum of all the torques acting on an object.
- Again, while this is important for rotational equilibrium, it does not directly relate to the normal force on the supports.

In conclusion, the correct answer is option 'A' because making the vertical sum of the forces equal to zero ensures that the object is in vertical equilibrium and the normal force on the supports is balanced with the weight of the object.