Be₄, F₉, Si₁₄, K₁₉, Ca₂₀

Among these, which element has one electron in its outermost shell?

K₁₉ has one electron in its outermost shell.
The atomic number of K is 19, and hence its electronic configuration is 2,8,8,1. From there, we can easily say that it has only 1 electron in its outermost shell and has a valency of 1. 

It can easily donate one electron from its outermost shell, and hence it is a metal. It is an alkali metal and all alkali metals have 1 valence electrons.