Which of the following shows variable valency?

P shows variable valency.

Sodium, calcium, and magnesium belongs to the S-group so they do not show variable valency.

Depending on the nature of the reaction, some elements combine with other atoms to donate, take, or exchange electrons in varying amounts. 

For phosphorus the electronic configuration is are 2,8,5. Phosphorus can either accept 3 electrons or donate 5 electrons to form a stable octet configuration.  

The valency is an atom is capacity for combining. Therefore phosphorus  has a valency of 3 and 5.