The smallest continent is Australia. Therefore, the correct answer is (A) Australia. Australia is both a country and a continent, and it is often referred to as the "smallest continent" due to its relatively small landmass compared to other continents.

(A) Keratin

Hair is primarily composed of a protein called keratin. Keratin is a fibrous structural protein that forms the main component of hair, as well as nails and the outer layer of the skin. It provides strength and resilience to these structures. The other options—Casein, Globulin, and Mucin—are proteins found in other parts of the body and have different functions.