Final answer:
The least acidic proton would be one not on a positively charged species, not near a carbonyl group, not the first removed in a polyprotic acid, and lacking resonance or inductive stability effects, typically in an alkane structure.
Step-by-step explanation:
To determine which proton is the least acidic, one must consider factors like charge, resonance, inductive effect, and the paricipation of the atom bearing the proton in hydrogen bonding. A proton H¹ on a positively charged species is more easily removed (thus, more acidic) compared to the same species in a neutral state due to the stabilization of the resulting species after deprotonation.
Protons adjacent to a carbonyl group (C=O) in aldehydes, ketones, and carboxylic acid derivatives are more acidic (pKa ~20) than those in alkanes (pKa ~50) because of resonance stabilization of the conjugate base. In polyprotic acids, the first proton to be removed is the most acidic as it's easier to remove a proton from a neutral molecule than from a negatively charged ion, resulting in subsequent protons being less acidic and possessing higher pKa values.
Considering these principles, the least acidic proton would be one that is not on a positively charged species, not adjacent to a carbonyl group, not the first proton removed from a polyprotic acid, and not significantly stabilized by resonance or inductive effects. It would typically be one that is part of an alkane structure without additional electronegative atoms or groups nearby to stabilize the conjugate base.