Well there has to be a right angle in there somewhere as the pole must be perpendicular to the ground.
Using the sine rule:
sinA/a = sinB/b where a = 10, A = 90, b = 5√3, B = B
Re-arrange for B, B = arcsin (b(sinA)/a)
B = arcsin(5√3(sin90)/10)
B = 60 degrees.
Answer is C