Question

plzzzzzzz answer with the reason

Answer 1

the answer is b)
this is because they have asked us the time when the horizontal velocity becomes equal to the vertical velocity so we would consider u= usina, while the final velocity v= ucosa ( since it has became equal to the vertical vel. in magnitude) and putting this in the equation
$v = u - gt$
(since the projectile motion is under gravity and its vertical vel. i.e. usina will be variable throughout the motion)

now..

$u \cos( \alpha ) = u \sin( \alpha ) - gt \\ \\gt = u \sin( \alpha ) - u \cos( \alpha ) \\ t = u \div g( \sin( \alpha ) - \cos( \alpha ) )$

Hope it helps!