**Explanation:**

When an insulator falls on the field circuit of a DC generator, it breaks the circuit and interrupts the flow of current in the field winding. This has a significant impact on the operation of the generator.

Here's a detailed explanation of why option 'A' is the correct answer:

**1. Field circuit in a DC generator:**
In a DC generator, the field circuit consists of the field winding and the field coils. The field winding is responsible for creating a magnetic field when a current flows through it. This magnetic field is essential for the generation of electricity in the armature winding.

**2. Role of the field circuit:**
The field circuit in a DC generator is responsible for creating a magnetic field that interacts with the armature winding. This interaction induces a voltage in the armature winding, which generates electric power.

**3. Impact of the insulator falling on the field circuit:**
When an insulator falls on the field circuit and breaks it, the flow of current through the field winding is interrupted. As a result, the magnetic field that is necessary for the generation of electricity in the armature winding is no longer present.

**4. Consequences of the broken field circuit:**
When the field circuit is broken, the DC generator loses its ability to generate electricity. Without the magnetic field, there is no induction of voltage in the armature winding, and therefore, no electric power is produced.

**5. Effect on the rotation of the generator:**
Since no electric power is being generated, the mechanical energy required to rotate the generator is not converted into electrical energy. As a result, the generator will experience a sudden loss of mechanical load and will stop rotating in a few rounds.

**6. Why the other options are incorrect:**
- Option 'B': The generator cannot continue to run as a DC motor without a functioning field circuit. The field winding is necessary to create the magnetic field required for the motor operation.
- Option 'C': Similarly, the generator cannot continue to run as a motor without the magnetic field produced by the field winding.
- Option 'D': None of the mentioned options are correct because the generator will stop running due to the broken field circuit.

In conclusion, when an insulator falls on the field circuit of a DC generator, it breaks the circuit, interrupts the flow of current in the field winding, and ultimately causes the generator to stop in a few rounds.