Calcium, Magnesium, Sulphur and Chlorine have atomic numbers 20, 12, 16, 17 respectively. Which of these does not show a valency of 2?

Based on the group number value we can identify the valency of elements.

$\text{Ca\hspace{0.17em}}\&\text{Mg}\to$ Belongs to IIA group; valency  = 2

$\text{S}\to$ Belongs to VI A groups and its valency  $8-6=2$

$\text{C}l\to$Belongs to VIIA group and its valency  $8-7=1$