Which statement best defines a Bronsted-Lowry base?

• A. It donates an H+ ion.
• B. It accepts an H+ ion.
• C. It accepts an electron pair.
• D. It donates a hydroxide ion.

Answer: (B)

Proton transfer is the hallmark of Bronsted-Lowry bases: they accept a proton (H+). In a reaction, the acid donates H+ and the base accepts it, forming the conjugate base and conjugate acid. For example, ammonia accepts a proton to become ammonium, and water can act as a base by accepting a proton to form hydronium. This is why the statement identifying a base as something that accepts an H+ ion is the correct description. The other ideas describe different notions: donating a proton defines an acid, not a base; accepting an electron pair is the Lewis-base concept; donating a hydroxide ion fits more with Arrhenius-type base behavior and is not the general Bronsted-Lowry definition.