Answer: C. 50
==============================================
Explanation:
The diagonal cuts the rhombus into two congruent isosceles triangles. We know they are isosceles because the non-diagonal sides are equal in length (since all four sides of a rhombus are the same length).
Let x be the measure of angle 1. This is one base angle. The other base angle is also x as well. The third angle of the bottom triangle is angle 3, which is given to us at 80 degrees. For any triangle the three angles always add to 180.
x+x+80 = 180
2x+80 = 180
2x = 180-80
2x = 100
x = 100/2
x = 50
Angle 1 is therefore 50 degrees.
Angle 2 is also 50 degrees because angles 1 and 2 are congruent alternate interior angles. Any rhombus is a parallelogram (but not the other way around) so the top and bottom lines of the rhombus are parallel, allowing the alternate interior angles to be congruent.
