The problem asks us to complete the sentence: "This is an acute angle because its measure is [less than/greater than] $90^{\circ}$."

1. Problem Description

The problem asks us to complete the sentence: "This is an acute angle because its measure is [less than/greater than] $90^{\circ}$."

2. Solution Steps

An acute angle is an angle whose measure is greater than $0^{\circ}$ and less than $90^{\circ}$. Therefore, an acute angle is less than $90^{\circ}$.

3. Final Answer

less than