Hi there, so our first order of business in order to solve this is to understand the definitions at hand.

Acute means the angle is less than 90 degrees.

Obtuse means that the angle is greater than 90 degrees.

A right angle is 90 degrees, it looks like the connections of a cross.

Equilateral means all the sides are the same length.

Now that we have that out of the way, we can look at our picture. Because nothing is labelled, we cannot assume anything about the measures of the sides, so equilateral is out of the question.

Since there is obviously no obtuse angle(think about your knee being slightly bent, that is an obtuse angle), we can rule out obtuse.

Because none of the angles are 90 degrees, we do not have a right triangle.

Therefore, we arrive at it being an acute triangle. So first we check and see if it satisfies the condition for an acute triangle(no angle 90 or greater). So I ask myself, does this look like my elbow fully bent for every angle? Yes. Therefore, the triangle is acute, and the solution is a.