Hydrogen resembles halogens in many respects for which several factors are responsible. Of the following factors, which one is most important in this respect?

The correct option is C Its tendency to gain a single electron in its valence shell to attain stable electronic configuration.
Electronic configuration of hydrogen is 1st. Thus, hydrogen is a single electron species that needs one extra electron to complete its octet and to attain stable electronic configuration.

Similarly, general outermost shell electronic configuration of halogen is $n{s}^{2}n{p}^5$. So, halogens also need one extra electron to complete their octet and to attain stable electronic configuration.

Thus, hydrogen and halogens both have tendency to gain an electron and attain stable electronic configuration and this is one of the most important aspects of resemblance of hydrogen with halogens.

Hence, option(B) is correct