The diamagnetic molecules are:

C₂,N₂,F₂ has no unpaired electron in their molecular orbital configuration representation.

$B_2 : \sigma 1s^2\sigma *1s^2\sigma 2s^2\sigma *2s^2\pi 2p_y^1\pi 2p_z^1$ : paramagnetic

$C_2 : \sigma 1s^2\sigma *1s^2\sigma 2s^2\sigma *2s^2\pi 2p_y^2\pi 2p_z^2$ : diamagnetic

$N_2 : \sigma 1s^2\sigma *1s^2\sigma 2s^2\sigma *2s^2\pi 2p_y^2\pi 2p_z^2\sigma 2p_x^2$ : diamagnetic

$O_2 : \sigma 1s^2\sigma *1s^2\sigma 2s^2\sigma *2s^2\sigma 2p_x^2\pi 2p_y^2\pi 2p_z^2\pi *2p_y^1\pi *2p_z^1$ : paramagnetic

$F_2 : \sigma 1s^2\sigma *1s^2\sigma 2s^2\sigma *2s^2\sigma 2p_x^2\pi 2p_y^2\pi 2p_z^2\pi *2p_y^2\pi *2p_z^2$ : diamagnetic