Answer:
a. fluorine
Explanation:
Fluorine is the element of group 17 and period 2. The electronic configuration of the element is .
Stable oxidation state = -1 of fluorine as it gains one electron to gain noble gas configuration.
With alkali metals, which have oxidation state of +1 form ionic compound of the form, MX where X is F.
Among the halogens, fluorine forms the most stable halide because of the comparable size of the hydrogen and fluorine. Thus, it is the weakest acid when compared with other hydrogen halides.
Fluorine is the most reactive in the halogen series and thus, combines with most of the elements.
Fluorine forms inter-halogen compounds of form XA only. Example - ClF.
Hence, option a is correct.