The side opposite to the right angle in a triangle is called the _________.

The correct option is A

hypotenuse

$\text{Here,}\angle B={90}^o.$
$\text{The side opposite to}\angle B\text{is AC}\text{which is the longest side.}$

$\text{The longest side of a right-angled}\text{triangle is called the hypotenuse.}$