Final answer:
The -C2H5 group, known as an ethyl group, is the only functional group listed capable of exclusively hydrophobic interactions, as it is nonpolar and does not mix well with water.
Step-by-step explanation:
The functional group that is capable ONLY of hydrophobic interactions from the given options is -C2H5, which is an ethyl group, a type of alkyl group. Alkyl groups, such as the ethyl group, are nonpolar and repel water molecules, which means they engage in hydrophobic interactions. These groups are soluble in lipids and fats and do not interact well with water or other polar substances.
The other functional groups listed: -COOH (carboxyl), -OH (hydroxyl), and -NH (amino) have polar characteristics or can ionize to form charged particles, which allows them to have hydrophilic interactions. Specifically, the carboxyl group ionizes to release hydrogen ions, becoming negatively charged, and the amino group can gain a proton to become positively charged, both contributing to their hydrophilic nature.