What is needed to standardize NaOH?

A primary standard must be:

• very pure,
• stable in air,
• non-hygroscopic,
• of known formula,
• easy to weigh accurately.

Since NaOH is a base, we standardize it using a suitable primary standard acid.

Check each option

A. Ferrous ammonium sulfate

• This is used in redox titrations, not for standardizing NaOH.
• So, not suitable.

B. dil. HCl

• Dilute HCl is not a primary standard because HCl is volatile and its concentration cannot be determined just by weighing accurately.
• So, not suitable.

C. Oxalic acid

• Oxalic acid (commonly as hydrated oxalic acid) is a well-known primary standard acid.
• It is commonly used to standardize NaOH solution.
• So, suitable.

D. Sodium tetraborate

• Sodium tetraborate (borax) is a primary standard base, generally used for standardizing acids, not NaOH.
• Since NaOH is also a base, this is not used for standardizing NaOH.

Conclusion

The correct primary standard used for standardizing NaOH is: $\boxed{\text{Oxalic acid}}$

Comparison with stored correct answer

Stored correct answer: C

Derived answer: C

Hence, the derived answer agrees with the stored correct answer.