In the above figure, let AB be the tower and C be the point on the ground 30 m away from the foot of AB from where the point A is observed. Join B, C and A, C. We get a right-angled triangle ABC with right angle at B. BC = 30 m. Also, the angle of elevation of the top of the tower AB from C is 30°. So, ∠ACB = 30°. We need to find the height of the tower, that is, AB.

In ∆ABC,

or,

or,

Hence the correct choice is B.