Insolubility of oxides and hydrolysis products in water and NaOH Steps:

• Identify initial white solids (Q): Na2O dissolves to NaOH; MgO forms insoluble Mg(OH)2; Al2O3 insoluble; SiO2 insoluble; NaCl, MgCl2, AlCl3 dissolve (AlCl3 hydrolyzes partially, clear solution); SiCl4 hydrolyzes to insoluble Si(OH)4. Thus, 4 beakers (MgO, Al2O3, SiO2, SiCl4).
• After excess NaOH(aq): Mg(OH)2 remains insoluble; Al2O3 and SiO2 unchanged (insoluble in dilute aqueous NaOH); Si(OH)4 dissolves to Na2SiO3; MgCl2 forms insoluble Mg(OH)2; AlCl3 forms then redissolves Al(OH)3 to aluminate; others clear. Thus, 4 beakers (MgO, Al2O3, SiO2, MgCl2). Why D is correct:
• Matches Q=4 initial insoluble/hydrolysis products and R=4 final insoluble in dilute NaOH(aq), per solubility rules for these compounds. Why the others are wrong:
• A: Underestimates both (ignores SiCl4 initial, MgCl2 final).
• B: Underestimates Q (ignores SiCl4), correct R not 2.
• C: Correct Q but underestimates R (ignores MgCl2 forming Mg(OH)2).

Final answer: D