A cylindrical coil of radius r, having n turns per unit length and carrying a current I has a magnetic field near its one end equal to:

$\frac{μ_0n I}{2}$

The correct answer is Option (1) → $μ_0n I$

The magnetic field B inside a solenoid is -

$B=μ_0nI$

where,

I = Current flowing through solenoid

n = Number of turns per unit length

However, Near the end of the solenoid, the field reduces because the field lines spread out.

The field at the end is exactly half the field inside.

$B=\frac{μ_0n I}{2}$