Identify the nature of the mixture

We need to separate a mixture of only $100\,\text{mg}$ containing:

• $p$-nitrophenol
• picric acid

Both are aromatic nitro compounds and both are fairly high-boiling, non-volatile solids. Also, picric acid is strongly polar due to three nitro groups and a phenolic $-OH$, while $p$-nitrophenol is also polar but less so.

Check each technique

Option A: Steam distillation

Steam distillation is suitable for compounds that are:

• steam volatile
• immiscible with water
• stable at steam temperature

Neither $p$-nitrophenol nor picric acid is appropriately separated by steam distillation, especially picric acid being highly polar and non-volatile.

So, A is not suitable.

Option B: $2$-$5$ ft long column of silica gel

Column chromatography on silica gel can separate such compounds, but for only $100\,\text{mg}$ sample, using a long column is not the most appropriate method. This is more material- and time-intensive and less convenient for such a small quantity.

So, B is not the best choice.

Option C: Sublimation

Sublimation works when one component sublimes easily and the other does not.

These compounds are not ideal for separation by sublimation; picric acid and $p$-nitrophenol are not a standard pair for efficient sublimation-based separation.

So, C is not suitable.

Option D: Preparative TLC

Preparative thin layer chromatography is especially useful when:

• the sample quantity is small
• compounds have different adsorption on silica/alumina
• separation of closely related organic compounds is needed

Since the mixture is only $100\,\text{mg}$ and both compounds differ in polarity, preparative TLC is the most appropriate technique.

Conclusion

The most appropriate method is: $\boxed{\text{D: Preparative TLC}}$

Comparison with stored correct answer

Stored correct answer: $\text{D}$

My derived answer: $\text{D}$

Therefore, they agree.