(d) When a particle is projected with a speed $v$ at $45{^\circ}$ with the horizontal then velocity of the projectile at maximum height.
$v^{'} = v\cos{45{^\circ} = \frac{v}{\sqrt{2}}}$
Angular momentum of the projectile about the point of projection
$= mv'h$
$= m\frac{v}{\sqrt{2}}h = \frac{mvh}{\sqrt{2}}$