Extension proportional to length under Hooke's law Steps:

• Calculate extension per unit length for X from first setup: 1.5 mm / 3 m = 0.5 mm/m.
• Calculate extension per unit length for Y from first setup: 1.0 mm / 1 m = 1.0 mm/m.
• For second setup, X extension: 0.5 mm/m × 1 m = 0.5 mm.
• For second setup, Y extension: 1.0 mm/m × 3 m = 3.0 mm; total = 0.5 mm + 3.0 mm = 3.5 mm. Why B is correct:
• Hooke's law: extension δ = (F L)/(A E), so for constant F and A, δ ∝ L, allowing scaling from first setup. Why the others are wrong:
• A: 1.5 mm - Matches only original X extension, ignores Y's contribution.
• C: 4.8 mm - Arises from erroneous ratio like (1.5+1.0)×(3+1)/4, not proportional scaling.
• D: 5.0 mm - Assumes equal extension rates for X and Y, contradicting given data.

Final answer: B