The 20m side is AB, and the 30m side is AC.

The vertical line passing through B on the AC side intersects with the AC extension line at point D.

Angle bad = 180- Angle BAC = 180- 150 = 30 degrees.

In the right triangle ABD: BD = AB/2 = 10m, because the angle BAD = 30 degrees.

Therefore, the area of the triangle ABC = base * height /2 = 30 * 10/2 = 150 square meters.

It takes at least 150a to buy this kind of turf.

Therefore, we should choose C.