We can answer this question by writing the electronic configuration of each ion then choosing the ones having six electrons in outermost d level. The number of electrons in the neutral atom can be known from the periodic table and the number beside the positive sign in the ion indicates how many electrons the atom lost (so you can calculate the number of electrons in the ions easily).
I hope these ions are in your choices:
(1) Os^2+:
It has the following configuration: [Xe] 4f^14 5d^6
(2) Ru^2+:
It has the following configuration: [Kr] 4d^6
(3) Zn^2+:
It has the following configuration: [Ar] 3d^10
(4) Mn2+:
It has the following configuration: [Ar] 3d^5
(5) Tc^2+:
It has the following configuration: [Kr] 4d^5
(6) Y^2+:
It has the following configuration: [Kr] 4d^1
(7) Fe2+:
It has the following configuration: [Ar] 3d^6
From the above configurations, the right choices are:
Os^2+
Ru^2+
Fe2+
