Which of the following species does not exist under normal condition ?

Key Idea Molecules with zero bond order , do not exist.
According to molecular orbital theory,
(a)$B{e}_2^{+}$(4+4-1=7)
$=\sigma 1s^2,\stackrel{\star }{1}{s}^2,\sigma 2s^2,\stackrel{\star }{2}{s}^1$
Bond order (BO) $=\frac{4-3}{2}=0.5$
(b)$B{e}_2^{+}$(4+4=8)
$=\sigma 1s^2,\stackrel{\star }{1}{s}^2,\sigma 2s^2,\stackrel{\star }{2}{s}^2$
Bond order (BO) $=\frac{4-4}{2}=0$
(c )$B_2^{+}$(5+5=10)
$=\sigma 1s^2,\stackrel{\star }{1}{s}^2,\sigma 2s^2,\stackrel{\star }{2}{s}^1,\pi 2p_x^1\approx \pi 2p_y^1$
Bond order (BO) $=\frac{6-4}{2}=1$
(d)$L{i}_2^{+}$(3+3=6)
$=\sigma 1s^2,\stackrel{\star }{1}{s}^2,\sigma 2s^2$
Bond order (BO) $=\frac{4-2}{2}=1$
Thus, $B{e}_2$ does not exist under normal conditions.