SAT Question 42b4493b: Lines, Angles, and Triangles answer and explanation with Desmos solution

Choice B is correct. The sum of the measures of the interior angles of a triangle is $180$ degrees. Since the triangle is a right triangle, it has one angle that measures $90$ degrees. Therefore, the sum of the measures, in degrees, of the remaining two angles is $180-90$, or $90$. It's given that the measure of one of the acute angles in the triangle is $51$ degrees. Therefore, the measure, in degrees, of the other acute angle is $90-51$, or $39$.