Oxidation number changes for hydrogen in the reaction

Steps:

• In AlH₃, Al is +3 and each H is -1; in 3H₂O, each H is +1 and O is -2.
• In Al(OH)₃, Al is +3, each O is -2, and each H is +1; in 3H₂, each H is 0.
• Al and O show no change; H from AlH₃ changes from -1 to 0 (increases by 1), while H from H₂O changes from +1 to 0 (decreases by 1).
• The reaction involves both oxidation (H increase) and reduction (H decrease) by 1 each.

Why C is correct:

• It matches the redox changes where one set of H atoms decreases by 1 (from +1 in H₂O to 0 in H₂) and another increases by 1 (from -1 in AlH₃ to 0 in H₂), per oxidation number rules.

Why the others are wrong:

• A: No element decreases by 2; Al remains +3.
• B: No dual decreases; one H set increases.
• D: Lacks "also," misphrasing the dual changes; C precisely describes both.

Final answer: C