Step 1: Recall the Bronsted-Lowry idea.
A Bronsted-Lowry acid donates a proton ($H^+$); a base accepts one. We watch how each species changes.
Step 2: Write the reaction.
\[ NH_3 + H_2O \rightleftharpoons NH_4^+ + OH^- \]
Step 3: Track the ammonia.
$NH_3$ becomes $NH_4^+$ by gaining a proton, so ammonia acts as the base.
Step 4: Track the water.
$H_2O$ becomes $OH^-$ by losing a proton. Losing $H^+$ is exactly what an acid does.
Step 5: Name water's role.
Because water donates a proton here, it behaves as a Bronsted-Lowry acid in this particular reaction.
Step 6: Choose the option.
So the correct description is a Bronsted-Lowry acid, option 2.
\[ \boxed{\text{A Bronsted-Lowry acid}} \]