Elements with valency 1 are:

The elements with valency +1 are always metals present in the left most corner of the periodic table in group I. They have only one electron in their outermost shell which they could lose easily to get an outermost shell with the maximum number of allowed electrons. But a -1 valency could be shown by the group 17 nonmetals also which have 7 electrons in their outermost shell and require only one electron to complete their octet.

Hence, the correct option is C.