Which of the following element will have valency one

The given four elements will have valency 1. Because all these elements have same valency configuration ${\mathrm{ns}}^1\& {\mathrm{ns}}^2{\mathrm{np}}^5$

$\mathrm{Potassium}\left(\mathrm{K}\right)-\left[\mathrm{Ar}\right]4{\mathrm{s}}^1$

$\mathrm{Chlorine} \left(\mathrm{C}\right)-\left[\mathrm{Ne}\right]3{\mathrm{s}}^{2}3{\mathrm{p}}^5$

$\mathrm{Hydrogen} \left(\mathrm{H}\right) -1{\mathrm{s}}^1$

$\mathrm{Fluorine} \left(\mathrm{F}\right) -\left[\mathrm{He}\right]2{\mathrm{s}}^{2}2{\mathrm{p}}^5$

Note: VIIA group elements exhibits valency 1, because they are just one electron sort for stable configuration (Octet number).