Which of the following elements exhibits variable valency due to d-orbital involvement?
d-block elements exhibit variable valency due to partially filled d-orbitals. Cu (group 11) shows this, unlike Na (s-block), Al (p-block), or Kr (p-block, full).
Ref: NCERT Class 11 Chemistry > Chapter 3: Classification of Elements and Periodicity in Properties > Topic: Electronic Configuration and Types of Elements - s p d f Blocks