Answer
d. $\dfrac{\sqrt{10}}{10}$
Work Step by Step
$\cos{\theta} = \dfrac{x}{r}$
For the point $(1,-3)$:
$$x = 1 \hspace{20pt} y = -3$$
$$r = \sqrt{x^2+y^2} \\ = \sqrt{(1)^2+(-3)^2} = \sqrt{10}$$
$\cos{\theta} = \dfrac{x}{r} = \dfrac{1}{\sqrt{10}} = \boxed{\dfrac{\sqrt{10}}{10}}$
