The colour of the ${\mathrm{X}}_2$ molecules of group 17 elements changes gradually from yellow to violet down the group. This is due to

IF Consider, $X_2\text{ as }{F}_2$

$\begin{array}{l}\to F_2\to MoT\text{ config is }\sigma /s^2{\sigma }^{\ast }/s^2\sigma 2s^2{\sigma }^{\ast }2s^2{\sigma }^{\ast }2p_z^2\pi 2p_x^2=\pi 2p_y^2\\ {\pi }^{\ast }2p_x^2=\pi \ast 2p_y^2\to \text{ HOMO }{\sigma }^{\ast }2p_z^0\to LuMo\end{array}$

Þ if gap between HOMO & LUMO Due to absorbed energy decreases

Þ complimentary colour is seen

Þ light energy is seen

Þ both ‘B’ & ‘C’ are thus applicable