A block A can slide on a frictionless incline of angle θ and length...

Since the elevator is moving upwards with a uniform velocity $v$, its acceleration is zero. Therefore, there is no pseudo force acting on the block, and the effective acceleration due to gravity inside the elevator remains $g$.

The component of acceleration of the block along the frictionless incline is $a = g \sin \theta$.

The block is released from rest, so its initial velocity is $u = 0$. Let $t$ be the time taken to slide down the incline of length $l$.

Using the second equation of motion, $s = ut + \dfrac{1}{2}at^2$:

$l = 0 + \dfrac{1}{2}(g \sin \theta)t^2$

$t^2 = \dfrac{2l}{g \sin \theta}$

$t = \sqrt{\dfrac{2l}{g \sin \theta}}$