Which molecule acts as a base in the Brønsted-Lowry theory?

In the Brønsted-Lowry theory, a base is defined as a substance that accepts protons (H⁺ ions). Therefore, the correct answer identifies a molecule as a proton acceptor. When a base encounters an acid in this framework, it effectively takes on a proton, resulting in the formation of a conjugate acid. This behavior reflects the concept that bases do not necessarily need to be neutral; rather, they play the role of accepting protons in a chemical reaction.

The other choices involve different characteristics that do not align with the definition of a Brønsted-Lowry base. For example, a proton donor is reflective of an acid rather than a base. Similarly, a neutral molecule can have various roles in chemical reactions but does not specifically indicate any tendency to accept protons unless it is named as a base by structure. Lastly, a molecule with a positive charge is not inherently a base; it may not have the capacity to accept protons depending on its specific chemical structure and context. Thus, the identification of a molecule as a proton acceptor is key to understanding its role as a base in aqueous chemistry according to the Brønsted-Lowry theory.