Answer:
The H in the carboxyl group.
Explanation:
Acetic acid can be written as CH₃COOH, where -COOH is the functional group carboxyl, responsible for the acidity of organic acids. The H in the carboxyl group is the one that is donated in the acid reaction.
CH₃COOH(aq) + H₂O(l) ⇄ CH₃COO⁻(aq) + H₃O⁺
Acetic acid is a weak acid, so just a small fraction of the molecules undergo this reaction to donate their hydrogen.
First a balanced reaction equation must be established:

→

Now if mass of aluminum = 145 g
the moles of aluminum = (MASS) ÷ (MOLAR MASS) = 145 g ÷ 30 g/mol
= 4.83 mols
Now the mole ratio of Al : O₂ based on the equation is 4 : 3
[
4Al +
3 O₂ → 2 Al₂O₃]
∴ if moles of Al = 4.83 moles
then moles of O₂ = (4.83 mol ÷ 4) × 3
=
3.63 mol (to 2 sig. fig.)
Thus it can be concluded that
3.63 moles of oxygen is needed to react completely with 145 g of aluminum.
A is the answer
Hope it helps :)
that's the answer I think