Since sin x = cos 90 - x for any angle x, we can write:
sin x = cos (90 - x)
Given sin x = cos 50°, we can substitute to get:
cos (90 - x) = cos 50°
Taking the inverse cosine of both sides, we get:
90 - x = 50°
Solving for x, we get:
x = 40°
Therefore, the answer is option A: 40°.