Physical Pharmacy

Step 1: Understanding diffusible substances.
For mixtures preparation, a diffusible substance is one that can easily disperse or spread out in a mixture. The diffusibility of a substance plays an important role in homogeneity.
Step 2: Analyzing the options.
(A) Caffeine citrate: Correct — Caffeine citrate is a diffusible substance used in pharmaceutical preparations, especially in mixtures where even distribution is required.
(B) Atropine sulphate: This is not as diffusible as caffeine citrate.
(C) Borax: Borax is typically used in other preparations but is not known for its diffusibility in mixtures.
(D) Bismuth carbonate: Bismuth carbonate is not a typical diffusible substance for mixtures preparation.
Step 3: Conclusion.
The correct answer is (A) Caffeine citrate, which is used for its ability to diffuse in mixtures.