Answer
a. $0$
Work Step by Step
$\cos{\theta} = \dfrac{x}{r}$
As can be seen from the figure as $\theta$ goes from $0^\circ$ to $90^\circ$, $x$ decreases and approaches zero.
At $x=0 \hspace{20pt} \cos{\theta} = \dfrac{0}{r} = 0$
$\therefore \cos{\theta}$ approaches $0$ as $\theta$ increases from $0^\circ$ to $90^\circ$
