To achieve perfect balance of primary and secondary forces as well as couples, the angle between the two cylinder V line engines should be 90 degrees. Let's understand why this is the correct answer.

Primary Forces:
Primary forces are the forces that act along the line of stroke and are balanced by the inertia forces of the reciprocating parts. In a V line engine, primary forces are generated due to the reciprocating motion of the pistons. These forces can be resolved into two components - one along the line joining the crankshafts of the two engines (Fp1) and the other perpendicular to this line (Fp2).

When the angle between the two crankshafts is 90 degrees, the component of primary forces along the line joining the crankshafts (Fp1) will be equal in magnitude and opposite in direction for both engines. This results in perfect balance of primary forces.

Secondary Forces:
Secondary forces are the forces that act perpendicular to the line of stroke and are balanced by the inertia forces of the reciprocating parts. In a V line engine, secondary forces are generated due to the angularity of the connecting rod. These forces can also be resolved into two components - one along the line joining the crankshafts of the two engines (Fs1) and the other perpendicular to this line (Fs2).

When the angle between the two crankshafts is 90 degrees, the component of secondary forces perpendicular to the line joining the crankshafts (Fs2) will be equal in magnitude and opposite in direction for both engines. This results in perfect balance of secondary forces.

Couples:
Couples are the moments that tend to rotate the engine about its axis. In a V line engine, couples are generated due to the angularity of the connecting rod. These couples can be resolved into two components - one along the line joining the crankshafts of the two engines (C1) and the other perpendicular to this line (C2).

When the angle between the two crankshafts is 90 degrees, the component of couples perpendicular to the line joining the crankshafts (C2) will be equal in magnitude and opposite in direction for both engines. This results in perfect balance of couples.

Therefore, to achieve perfect balance of primary and secondary forces as well as couples, the angle between the two cylinder V line engines should be 90 degrees.