Bounded on one end by the tonsils, which are the primary defense during breathing.
a. the larynx
b. the nose
c. the pharynx

c. The pharynx

Explanation:

The tonsils (palatine tonsils) are structures made up of lymphoid tissue, rich in white blood cells. They are located in the oropharynx, ie, in the transition from the mouth to the pharynx. That is, the pharynx has an end limited by tonsils, which are the main defense during breathing.

The pharynx is shaped like a funnel (wide above and narrow below) 12 to 15 cm long and is located in front of the cervical spine, behind the nasal cavities, the oral cavity and the larynx and follows to the esophagus. The ears also have a connection with the pharynx.

The pharynx connects the nose and mouth to the larynx and esophagus. It works as a meeting point between the digestive and respiratory tract.

genetic variation can be caused by mutation (which can create entirely new alleles in a population), random mating, random fertilization, and recombination between homologous chromosomes during meiosis (which reshuffles alleles within an organism's offspring).