What is the difference between an assay and an extemporaneous preparation? Can an extemporaneous preparation be labelled as the BP mixture?

I came across this question: "Which of the following is bigger?" $$K^{+},Ca^{2+},Al^{3+},S^{2-},Cl^{-}$$ Now, I know that the radius decreases as the period progresses to the right and increases as we go down on the P.T. So I get to this: $$S^{2-}>Cl^{-}>K^{+}>Ca^{2+}$$ Also, I know that $Al^{3+}<...