Let's see, Cobalt 3+, a transition metal ion, meaning that it has no s orbital valence electrons, only d. This is because whenever transition metals form positive ions, they lose their s valence electrons first.
So, n=3 (since it is 3d valence electrons)
Now, l has the following possible values, 0, 1, and 2
0 is s, 1 is p, and 2 is d
Now, we just stated that the valence electrons are d orbital electrons.
Thus, d has to be 2, not 0 or 1.