Which one of the following is primarily responsible for conduction of current in a metal?

In metals, the outermost valence electrons are loosely bound and become delocalized, forming a 'sea' of free electrons that move throughout the metallic lattice. When a potential difference is applied, these free electrons drift in a particular direction, constituting the electric current. Bound electrons remain attached to atoms and do not contribute to conduction. Ions in a solid metal are fixed at lattice sites and do not move. Therefore, the conduction of current in a metal is primarily due to free electrons.