The strength of an aqueous NaOH solution is most accurately determined by titrating: (Note: consider that an appropriate indicator is used)

• Primary vs. Secondary Standards: Sodium hydroxide ($$\text{NaOH}$$) is hygroscopic and absorbs carbon dioxide ($$\text{CO}_2$$) from the air, making it a secondary standard. Its concentration cannot be calculated directly by weighing. It must be standardized against a stable primary standard like oxalic acid ($$\text{H}_2\text{C}_2\text{O}_4$$).
• Concentrated Acids: Concentrated $$\text{H}_2\text{SO}_4$$ is not a primary standard and requires dilution and prior standardization, making it unsuitable for an accurate direct titration reference.
• Apparatus Configuration: In standard laboratory practice, the highly alkaline, caustic sodium hydroxide solution is placed in the burette. This prevents the solution from sticking to or etching the glass of a volumetric pipette and allows the accurately measured primary standard (oxalic acid) to remain cleanly inside the conical flask.

Conclusion:

The concentration of the aqueous $$\text{NaOH}$$ solution is most accurately verified when it is placed in the burette and titrated against a known concentration of aqueous oxalic acid measured out into a conical flask.

Answer: Option B — Aq. NaOH in a burette and aqueous oxalic acid in a conical flask