1) for most substances, describe the relative densities of the phases.

There are three main phases of matter. Solids consist of atoms that are arranged in a regular crystal lattice. The atoms are closely spaced and hence the density is high. In a liquid, atoms are more loosely arranged, in no definite crystal arrangement, and kept together by weak intermolecular forces. The density is therefore less than a solid. In a gas, individual molecules/atoms only interact weakly and the density is even less than that of a liquid. 

"Reversibility" is the one principle among the following choices given in the question that is why the doctor recommends aquatic activity. The correct option among all the options that are given in the question is the fourth option or the last option. I hope that this is the answer that has come to your desired help.