In the equation #HCl(g) + H_2O(l) -> H_3O^+(aq) + Cl^(-)(aq)#, which species is a Bronsted-Lowry acid?
As you know, a Bronsted - Lowry acid is a proton donor, which means that you can identify a Bronsted - Lowry acid in a reaction by looking at which chemical species lost a proton.
A proton is simply a hydrogen ion,
In your case, you have
#color(red)("H")"Cl"_text((aq]) + "H"_2"O"_text((l]) -> "H"_3"O"_text((aq])^(color(red)(+)) + "Cl"_text((aq])^(-)#
Notice that hydrochloric acid,
The remaining chloride anion,
Since hydrochloric acid donated a proton, it acted as a Bronsted - Lowry acid.