In the following sets of compounds, the one which contains only medicinal compounds is

Check each set by identifying any non-medicinal substance. $\mathrm{Alizarin}$ is a dye, so $\mathrm{A}$ is not correct. $\mathrm{Phenolphthalein}$ is mainly an acid-base indicator, so $\mathrm{B}$ is not correct. In $\mathrm{C}$, boric acid is used as a mild antiseptic, chloramphenicol is an antibiotic, and aspirin is a medicine, so all three are medicinal compounds. In $\mathrm{D}$, $9$-oxdecanoic acid is not a medicinal compound. Therefore, after checking all options, the correct choice is $\mathrm{C}$.