(A) 1, 6, 2 and 3
Balancing: Al₂O₃ + 6HCl → 2AlCl₃ + 3H₂O. Al: 2=2, Cl: 6=6, O: 3=3, H: 6=6. So a=1, b=6, c=2, d=3.
Balance Al first (2 on LHS → c=2), then Cl (6 on RHS → b=6), then check H and O (6H gives 3H₂O). The correct coefficients are 1, 6, 2, 3 — option A. Watch out for option D which incorrectly doubles all terms.