The correct option is (c)
Explanation —
If $2 s-2 p$ mixing is not operative, then molecular orbitals may be arranged in order of energy as follows :
$\sigma 1 s, \quad \sigma^* 1 s, \quad \sigma 2 s, \quad \sigma^* 2 s, \quad \sigma 2 p_z, \quad \pi 2 p_x=\pi 2 p_y$, $\pi^* 2 p_x=\pi^{\star} 2 p_y, \sigma^* 2 p_z$
Applying this configuration, $\mathrm{Be}_2, \mathrm{~B}_2$ and $\mathrm{N}_2$ will be diamagnetic, but $C_2$ will be paramagnetic with electronic configuration :
$\sigma 1 s^2, \sigma^* 1 s^2, \sigma 2 s^2, \sigma^* 2 s^2, \sigma 2 p_z^2, \pi 2 p_x^1=\pi 2 p_y^1$
