Which technique among the following, is most appropriate in separation of a mixture of $$100 \text{ mg}$$ of $$p$$-nitrophenol and picric acid?

We need to find the most appropriate technique for separating a mixture of $$100 \text{ mg}$$ of $$p$$-nitrophenol and picric acid.

The total quantity of the mixture is $$100 \text{ mg}$$, which is a very small amount (milligram scale).

Steam distillation is used to separate volatile compounds that are immiscible with water. While $$p$$-nitrophenol is steam volatile, this technique is generally used for larger quantities and is not ideal for milligram-scale separations.

A 2-5 ft long column of silica gel is used for preparative column chromatography. This technique is suitable for separating gram-scale quantities (typically 1-100 g). For just $$100 \text{ mg}$$, using such a long column would result in significant loss of sample and poor separation efficiency.

Sublimation is used when one component sublimes and the other does not. Neither $$p$$-nitrophenol nor picric acid readily sublimes under normal conditions, so this technique is not appropriate here.

Preparative TLC is specifically designed for separating small quantities of compounds, typically in the range of $$1 \text{ mg}$$ to $$100 \text{ mg}$$. The mixture is spotted on a thick TLC plate, developed in a suitable solvent, and the separated bands are scraped off and extracted. This is the most appropriate technique for $$100 \text{ mg}$$ of a mixture.

Since the quantity is only $$100 \text{ mg}$$, preparative TLC is the most suitable technique for this separation.

Hence, the correct answer is Option D: Preparative TLC (Thin Layer Chromatography).