We use angle sum property of triangle here :
sum of three angles = 180 degrees
118.6 + 42.3 + third angle =180
160.9 + third angle =180
third angle =180-160.9
third angle =19.1 degrees
all Triangles have a complete measure of 180 degrees so 180- 118.6-42.3 =19.1
the answer is C!
