The requirements for one type of atom to substitute for another in a solid solution are:
All substitutions must be limited to the same element.
An atom must be similar in size.
The substituting atom must be from the same period.
The substituting atom must be from the same group.
An atom must be identical in size.

the atom must be similar in size and it must be from the same group.

Explanation:

A solid solution is essentially a multicomponent system which is composed of a mixture of two or more elements sharing the same crystal lattice. Alloys are solid solutions.

The requirement for one type of atom to substitute for another in a solid solution are two fold

1) The atom must be similar in size

2) The substituting atom must be from the same group.

it is b that is the answer.