The functional group most likely to gain a proton and become positively charged is the amino group (–NH₂ → –NH₃⁺).

Why the amino group?

  • An amino (amine) group contains a nitrogen atom with a lone pair of electrons that can accept a proton (H⁺), so it behaves as a base and becomes positively charged as –NH₃⁺.
  • A carboxyl group (–COOH) usually loses a proton to form –COO⁻, so it tends to become negatively charged, not positive.
  • A hydroxyl group (–OH) is only very weakly acidic/basic in biological conditions and does not usually pick up a proton to form a stable positively charged species.

So, if the options are hydroxyl, carboxyl, and amino, the correct choice is the amino group.

In many biology and chemistry textbook questions with this exact wording, the keyed answer is: “The amino group is most likely to gain a proton and become positively charged.”

TL;DR: Choose the amino group (–NH₂).