Let's start from the fact that the sum of interior angles of a regular polygon is always computed as \((n-2)*180°\), where n is the number of sides. Thus, the sum of interior angles of a pentagon is 3*180 = 540°, so that, because it is regular, each of its angles is 108° (540/5).

Then, at a vertex where two diagonals meet, the angle is divided in three equal portions, so that the angle between the two diagonals measures 108/3 = 36°.

Answer B