Answer
$\overrightarrow{CA} \perp\overrightarrow{DB}$
Work Step by Step
Here, ABCD is a rhombus having two diagonals abbreviated as $\overrightarrow{CA}$ and $\overrightarrow{DB}$.
The angle between two diagonals of a rhombus ABCD is calculated as: $\overrightarrow{CA}$ and $\overrightarrow{DB}$
$ \theta = \cos ^{-1} (\dfrac{\overrightarrow{CA} \cdot \overrightarrow{DB}}{|\overrightarrow{CA}||\overrightarrow{DB}|})=\cos ^{-1} (\dfrac{(\overrightarrow{DA})^2-(\overrightarrow{AB})^2}{ |\overrightarrow{CA}||\overrightarrow{DB}|})$
Since, ABCD is a rhombus, then $|\overrightarrow{DA}|=|\overrightarrow{AB}|$
or, $\theta=\cos ^{-1} (0)=\dfrac{\pi}{2}$ or $90 ^{\circ}$
This implies that $\overrightarrow{CA} \perp\overrightarrow{DB}$
