Commutation in a D.C. Machine:
Commutation is an important process in a D.C. machine, which refers to the reversal of current in the armature coil as the coil passes through the magnetic neutral plane. It ensures that the current flowing through the coil remains unidirectional, allowing for proper operation of the machine.

Counter to Commutation:
Among the given options, coil leakage induction (option A) is the factor that counters commutation in a D.C. machine. Let's understand why this is the correct answer.

1. Commutation Process:
Before discussing the counter to commutation, let's briefly understand the commutation process itself. As the armature coil rotates within the magnetic field, the current in the coil changes direction when it passes through the magnetic neutral plane. This change in current direction is achieved by the commutator and brushes.

2. Commutator and Brushes:
The commutator is a cylindrical rotor with segmented copper bars insulated from each other. The brushes, typically made of carbon, make contact with the commutator segments. The brushes are responsible for transferring current between the stationary external circuit and the rotating armature coil.

3. Role of Coil Leakage Induction:
Coil leakage induction refers to the magnetic flux that links with the armature coil but does not pass through the core of the coil. This leakage flux is caused by the imperfect magnetic circuit and is present in all practical machines. It is one of the factors that counter commutation in a D.C. machine.

4. Effect on Commutation:
When coil leakage induction occurs, it generates a back e.m.f. in the coil during commutation. This back e.m.f. opposes the applied voltage, making it difficult to reverse the current in the coil. As a result, the commutation process becomes less effective, leading to poor commutation.

5. Consequences of Poor Commutation:
Poor commutation can have several adverse effects on the performance of a D.C. machine, including:
- Increased sparking at the brushes: Poor commutation causes arcing and sparking at the brushes, leading to brush wear and reduced brush life.
- Reduced efficiency: Inefficient commutation results in increased power losses and reduced overall machine efficiency.
- Increased electromagnetic interference: Poor commutation can generate electromagnetic interference, affecting nearby electronic devices.

6. Mitigation Techniques:
To mitigate the adverse effects of coil leakage induction and improve commutation, various techniques are employed in D.C. machines. These include:
- Proper design of the magnetic circuit to minimize leakage flux.
- Using interpoles or compensating windings, which produce a magnetic field that counteracts the effects of coil leakage induction.
- Employing commutating poles, which assist in reversing the current in the armature coil during commutation.

Conclusion:
In summary, among the given options, coil leakage induction is the factor that counters commutation in a D.C. machine. It generates a back e.m.f. in the armature coil during commutation, making it more challenging to reverse the current. This can result in poor commutation, leading to sparking, reduced efficiency, and electromagnetic interference. To overcome this, techniques such as proper magnetic circuit design, interpoles, and commutating poles are employed.