2. Hydrophobic amino acids
Hydrophobic amino acids is amino acids are most likely present at the dimerization interface of stat3 proteins.
A dimerization is an addition reaction in which two molecules of the same compound react with each other to give the adduct.
Hydrophobic amino acids is the correct answer because the polar and charged amino acids most likely interact with water molecules in cytosol and would not be involved in protein-protein interactions.
In contrast, the side chains of hydrophobic amino acids are free and most likely participate in dimerization of STAT3.