Acute Angles
Different Angles have different names:
An Acute Angle is less than 90°
This is an acute angle
This is an acute angle
All the angles below are acute angles:
Which Angle?
Remember to look carefully at which angle you are being asked to name:
The acute angle is the small angle which is less than 90°.
If you choose the larger angle you would have a Reflex Angle instead: 

The smaller angle is an Acute Angle, but the larger angle is a Reflex Angle 