Answer
$PCl_{4}^+$ is the Lewis acid and $Cl^-$ is the Lewis base.
Work Step by Step
A Lewis acid is a chemical species with an empty orbital capable of accepting electrons from a Lewis base, and a Lewis base is a chemical species with a free electron pair (not bonding with any other atom). Therefore, $PCl_{4}^+$ is a Lewis acid and it reacts with $Cl^-$ (Lewis base) to form a Lewis adduct. The free electron pair has been highlighted in red.
