**Explanation:**

When a block of mass m kg is kept on a weighing machine in an elevator, the reading on the weighing machine is equal to the normal force exerted by the weighing machine on the block. This normal force is responsible for balancing the gravitational force acting on the block.

**1. Forces acting on the block in the elevator:**

In this scenario, the block is subjected to two main forces:

- Gravitational force (mg): This force acts vertically downward and is given by the product of the mass (m) and the acceleration due to gravity (g).

- Normal force (N): This force acts vertically upward and is exerted by the weighing machine to balance the gravitational force.

**2. Retarding upward acceleration of the elevator:**

In this case, the elevator is retarding upward, which means it is experiencing a negative acceleration (-a) in the upward direction. The negative sign indicates that the acceleration is opposite to the positive direction.

**3. Determining the reading on the weighing machine:**

To find the reading on the weighing machine, we need to consider the net force acting on the block. The net force is given by:

Net force = ma

Since the block is retarding upward, the net force can be expressed as:

Net force = (mg - ma)

**4. Calculating the normal force:**

The normal force exerted by the weighing machine is equal to the net force acting on the block. Therefore, we can write:

N = mg - ma

Factoring out the common term 'm' from both terms, we get:

N = m(g - a)

**5. Final expression for the reading on the weighing machine:**

The reading on the weighing machine is equal to the normal force (N). Substituting the value of N from the previous step, we get:

Reading on the weighing machine = m(g - a)

Simplifying this expression further, we have:

Reading on the weighing machine = mg - ma = m(1 - a/g)

Thus, the correct answer is m(1 - a/g).