The Quantity Called the "Enclosed Mass" M(<r)represents ALL of the Mass

The quantity called the "enclosed mass" M(<r) represents ALL of the mass inside radius r. If the enclosed mass M(<r) is constant with increasing radius, where is the mass?

A) concentrated in the center
B) distributed in the shape of a doughnut, with a hole in the center
C) spread out over a large volume
D) impossible to say