In the cylinder below, the radius, r, is 6 m, and the volume is 252m3. what is the altitude of the cylinder?

There is no 'cylinder below'.  But I think you've told us enough about it
so that we can answer your question.

The volume of a cylinder that's standing up is   

                                                   V = (pi) x (radius)² x (altitude) .

Fill in the numbers we know:  252 m³ = (pi) x (6 m)² x (altitude)

Divide each side by  (pi x 36 m²) :  (252 m³) / (pi x 36 m²) = altitude

                                                        7 / pi   m  =  altitude

                                                         2.228 m  =  altitude  (rounded)

The answer is part a: use

C