You need a list of elements to compare.
The procedure is to wirte the electron configuration and compare the number of electrons in the valence electron shell.
I will work with this list, to show you the procedure
(1) tin
(2) sulfur
(3) arsenic
(4) fluorine
element atomic number number of abbreviated electron
electrons configuration
tin 50 50 [Kr] 5s^2 4d^10 5p2
sulfur 16 16 [Ne] 3s^2 3p^4
arsenic 33 33 [Ar] 4s^2 3d^10 4p^3
fluorine 9 9 [He] 2s^2 2p^5
The valence electrons are those in the last shell.
So, fluorine has 2 + 5 = 7 valence electrons, while tin has 2 + 2 = 4, sulfur has 2 + 4 = 6 and aersenic has 2 + 3 = 5.
Therefore, the answer is fluorine.