A straight long wire of radius 'a' carries a current 'I' uniformly distributed along its cross-section. The magnetic field B in the region r < a will be proportional to

Correct option is B

The correct answer is: (b) r Explanation: Inside a uniformly current-carrying conductor (r < a), the magnetic field increases linearly with distance r from the center: B ∝ r (from Ampere’s law)
Information Booster:
· For r > a: B ∝ 1/r (outside wire).
· Inside conductor: current enclosed ∝ r².
· Ampere’s Law: ∮B·dl = μ₀I_enclosed.
· Magnetic field is zero at the center.