Answer:
Cl-
Step-by-step explanation:
The equation of the reaction is;
H+(aq)+Cl-(aq)+H2O(l)→H3O+(aq)+Cl-(aq)
Lewis base refers to substances that can donate a lone pair of electrons(e.g OH^-, Cl^- etc) or a pair of nonbonding electrons.(eg NH3, H20 etc). A Lewis base is therefore an electron-pair donor. They are easily recognized because they often have lone pairs or are negatively charged species.
If we look at the products, Cl- is a Lewis base because it is a lone pair donor. We see this more clearly because the specie is negatively charged.