Not All Protons in a Molecule Can Be Donated in an Acid-Base

Multiple Choice

Not all protons in a molecule can be donated in an acid-base reaction. Below are three molecules, each with the proton donated in an acid-base reaction circled. What do these protons have in common?

A) They are all attached to oxygen.
B) They are all in a carbon-containing molecule.
C) They are all attached to the most electronegative atom in the molecule.
D) They are the only hydrogens in the molecule.
E) They are all attached to carbon.