At what distance above the surface of the earth is the gravitational field 4.9 m/s^2

That's 1/2 of what it is on the surface.

The distance between the center of the Earth and any object
on the surface is 1 Earth radius ... about 3960 miles.

Gravitational force is inversely proportional to the square of
the distance between the centers of the objects, so in order
to reduce the acceleration of gravity by 1/2, you increase the
distance by √2 .

           (3960 miles) x (√2) = 5,600 miles from the center

                                         = 1,640 miles above the surface.

                                          

No it doesnt because it can be any number.

It should be d if not then c