One angle is a right angle. The measures of the other 2 angles are as follows:

Choice B is correct. The triangle shown is a right triangle, where the interior angle shown with a right angle symbol has a measure of $90°$. It's shown that the other two interior angles measure $13°$ and $a°$. The sum of the measures of the interior angles of a triangle is $180°$; therefore, $90+13+a=180$. Combining like terms on the left-hand side of this equation yields $103+a=180$. Subtracting $103$ from both sides of this equation yields $a=77$.

Choice A is incorrect. This is the measure, in degrees, of the other acute interior angle of the right triangle, not the value of $a$.

Choice C is incorrect. This is the measure, in degrees, of the right angle of the right triangle, not the value of $a$.

Choice D is incorrect. This is the sum of the measures, in degrees, of the other two interior angles of the right triangle, not the value of $a$.