Definition of a force couple Steps:

• Identify a couple as two forces producing pure torque (rotation) with no net force or translation.
• Require forces to be equal in magnitude for net force to be zero.
• Require forces to act in opposite directions along parallel lines for torque.
• Confirm they are not collinear to avoid cancellation.

Why D is correct:

• A couple's definition requires equal magnitudes to ensure zero net force, per Newton's laws.

Why the others are wrong:

• A: Forces act on parallel but separate lines, not the same straight line, which would cancel without torque.
• B: Opposite directions are required, but the key distinguishing feature is equal magnitude for balance.
• C: No requirement for "opposite type" (e.g., gravitational vs. magnetic); types can be the same.

Final answer: D