According to Molecular Orbital Theory:
C2²⁻ is expected to be diamagnetic
O2²⁺ is expected to have a longer bond length than O2
N²⁺ and N²⁻ have the same bond order
He²⁺ has the same energy as two isolated He atoms

(A) C2²⁻ bond order
Bond order = (Total no. of bonding electrons - Total no. of antibonding electrons) / 2
Total no. of electrons in C2²⁻ is 14
σ1S², σ1S², σ2S², σ2S², π2Px², π2Py²
Bond order = (10 - 4) / 2 = 3
If bond order is in integers then molecule/ion is diamagnetic
(B) Bond order α 1/Bond length
O2²⁺ bond order is 3
O2 bond order is 2
(C) N²⁺ and N²⁻ have the same bond order.