Of the species NO, NO+, NO2+ and NO-, the one with minimum bond strength is:

🧠 Bond strength ∝ bond order. Calculate BO for each species by counting total electrons (N = 7e, O = 8e).

| Species | Electrons | BO | |---|---|---| | $\ce{NO^{2+}}$ | 13 | (10−3)/2 = 3.5 (strongest) | | $\ce{NO+}$ | 14 | (10−4)/2 = 3 | | NO | 15 | (10−5)/2 = 2.5 | | $\ce{NO-}$ | 16 | (10−6)/2 = 2 (weakest) |

Each additional electron from $\ce{NO^{2+}}$ goes into an antibonding $\pi^*$ orbital, progressively weakening the bond. $\ce{NO-}$ has the lowest BO and thus the minimum bond strength.

$\boxed{\text{Answer: (d) } \ce{NO-}}$