Introduction:
In a DC shunt-generator, the electromagnetic torque is generated due to the interaction between the magnetic field produced by the field winding and the armature current. This torque is responsible for converting electrical energy into mechanical energy.

Explanation:
When the electromagnetic torque in a DC shunt-generator is opposite, it means that the direction of the torque is opposite to the direction of rotation. This situation indicates that the mechanical power is being absorbed by the machine rather than being delivered.

Reasoning:
To understand why the correct answer is option 'A' (Mechanical power is absorbed by the machine), let's consider the basic principle of operation of a DC shunt-generator.

- A DC shunt-generator consists of a field winding and an armature winding.
- When the field winding is excited with a direct current, it creates a magnetic field in the machine.
- The armature winding is connected to an external mechanical prime mover, such as a turbine or an engine.
- When the armature winding is supplied with a direct current, it produces an armature flux.
- The interaction between the field flux and the armature flux generates an electromagnetic torque.
- This torque causes the armature to rotate, converting electrical energy into mechanical energy.

Now, if the electromagnetic torque is opposite, it means that the torque is acting against the rotation of the machine. This implies that the prime mover has to exert more mechanical power to overcome the opposing torque and keep the machine running.

Since the prime mover is exerting more mechanical power, it indicates that the machine is absorbing mechanical power instead of delivering it. In other words, the machine is acting as a load or a consumer of mechanical power rather than a source of mechanical power.

Conclusion:
Therefore, when the electromagnetic torque in a DC shunt-generator is opposite, it can be concluded that the machine is absorbing mechanical power. This implies that the machine is acting as a load rather than a generator, and the prime mover has to supply additional power to keep the machine running.