Based on molecular orbital theory, which species has the highest bond order?

Based on molecular orbital theory, $O_2^{+}$ species has the highest bond order.
$O_2^{+}$ valence electrons = 6 + 6 – 1 = 11
$\left(\sigma 2s{)}^2\right(\sigma *2s{)}^2\left(\sigma 2p{)}^2\right(\pi 2p{)}^4({\pi }^{*}2p{)}^1$

BO = ½ × (6−1) = 2.5;
Hence, option A is correct.