Forces Forming a Couple: Equal, Opposite, Non-Collinear

Steps:

• Sum force vectors: resultant is zero if magnitudes equal and directions opposite.
• Confirm non-collinearity: forces act along parallel but offset lines to produce torque.
• Calculate moment: non-zero torque indicates rotation without translation.
• Eliminate options lacking both zero resultant and couple.

Why B is correct:

• Two equal, opposite forces act parallel but separated, yielding zero vector sum (Newton's first law for equilibrium) and non-zero moment arm for pure rotation.

Why the others are wrong:

• A: Forces unequal, so non-zero resultant causes translation.
• C: Equal opposite forces collinear, zero moment (no couple, just equilibrium).
• D: Forces same direction, non-zero resultant and no opposition for couple.

Final answer: B