BIf both Assertion and Reason are correct but Reason is not a correct explanation of the Assertion.
b
Magnetic field inside the solen oid at point $P$ is given by,
$\mathrm{B}=\frac{\mu_{0}}{4 \pi}(2 \pi \mathrm{ni})[\sin \alpha+\sin \beta]$
where $\mathrm{n}=\mathrm{no.}$ of turns per unit length
$=\mathrm{N} / \mathrm{l}$
Thus it is clear that mangetic field is independent of length and cross sectional area.
Also the magnetic field with in the solenoid is uniform and parallel to the axis of the solenoid.