1. What is being asked?

We need to determine the most accurate way to find the strength (concentration) of an aqueous $\text{NaOH}$ solution by titration.

For accurate acid-base titration, we should use:

• a standard solution in the conical flask or burette as appropriate,
• the solution whose concentration is to be determined in the burette if it is unstable,
• and avoid using unsuitable reagents like concentrated acids directly.

2. Important chemical consideration: NaOH is not a primary standard

Aqueous $\text{NaOH}$ is not accurately weighed and standardized directly because:

• it absorbs $\text{CO}_2$ from air,
• it is hygroscopic.

Hence, its concentration is best determined by titrating it against a primary standard acid, such as oxalic acid.

Oxalic acid is suitable because:

• it can be obtained pure,
• it is stable,
• its solution can be prepared accurately.

3. Which solution should be placed in the burette?

For best accuracy in this case:

• the unknown NaOH solution is usually taken in the burette,
• a known volume of standard oxalic acid is taken in the conical flask.

This is preferred because the NaOH solution is unstable on standing and should not be kept exposed unnecessarily; also standard oxalic acid can be measured accurately into the flask.

Reaction:

At equivalence:

so concentration of NaOH can be determined accurately.

4. Evaluate each option

Option A

Aq. NaOH in a pipette and aqueous oxalic acid in a burette

This means the unknown NaOH is measured by pipette and standard oxalic acid is delivered from burette.

This is less appropriate because the unstable NaOH solution is being measured as a fixed aliquot, whereas the standard solution should preferably be measured accurately and kept in flask, and NaOH is better run from burette during standardization.

So, A is not the most accurate.

Option B

Aq. NaOH in a burette and aqueous oxalic acid in a conical flask

This is the standard and most accurate setup:

• standard oxalic acid in conical flask,
• unknown NaOH in burette.

Thus, B is correct.

Option C

Aq. $NaOH$ in a volumetric flask and concentrated $H_2SO_4$ in a conical flask

This is not a titration arrangement at all. A volumetric flask is not used to deliver titrant during titration. Also concentrated $H_2SO_4$ is not used directly for such accurate acid-base titration.

So, C is incorrect.

Option D

Aq. $NaOH$ in a burette and concentrated $H_2SO_4$ in a conical flask

Although NaOH is in burette, concentrated $H_2SO_4$ is unsuitable for accurate direct titration here because concentrated sulfuric acid is not used this way as a standard aqueous titrant for precise determination of NaOH strength.

So, D is incorrect.

5. Final conclusion

The most accurate determination is done by titrating aqueous NaOH from the burette against standard aqueous oxalic acid in the conical flask.

Therefore, the correct option is: