b)  Trp is an aromatic amino acid with a large, primarly aromaic side chain.  Trp is therefore usually foud in the non-polar interior of a globular protein. Although Phe also has a non-polar aromatic side chain,  Phe may not be a suitable replacement for Trp because Van der Waals contacts with the benzene ring of Phe may be less favorable than with the indole ring of Trp resulting in subtle but significant alteration in protein structure.

c)  Lys and Arg both possess side chains of similar length and are polar.  Ile and leu are similar in size are both hydrophobic. In either case a substitution of one reside for the other results in only a small steric changes and causes little alteration in the overall conformation of the protein.

d)  Gly has the smallest side chain of any amino acid (R=H) and can fit into  regions of structure that accomodate no other residue (like type I and Type II turns). Substitution of any other residue in these instances would result in significant structural differences in the protein. Gly residues that are not subject to these constraints are more likely to be substituted for other residues.