Which of the following angles is acute?

An acute angle is defined as an angle that measures less than 90 degrees. Among the given options, 30 degrees is the only angle that fits this definition. It is clearly less than 90 degrees, confirming its classification as an acute angle.

The other angles do not meet the criteria for being acute. For instance, 90 degrees is classified as a right angle, which is not acute since it is equal to the threshold of 90 degrees. The angle measuring 120 degrees is obtuse, as it is greater than 90 degrees. Finally, 180 degrees is a straight angle, which is also not acute as it represents a complete half-turn. Thus, the only angle that qualifies as acute is indeed 30 degrees.