In which of the following structures does fertilization of an ovum by a spermatozoa occur?

a. Sodium: Correct. Sodium (Na⁺) is the primary extracellular cation and plays a crucial role in maintaining osmotic pressure and fluid balance in the body.

b. Magnesium: Magnesium (Mg²⁺) is important for many cellular functions but is not a major contributor to osmotic pressure compared to sodium.

c. Calcium: Calcium (Ca²⁺) is vital for bone health and cellular processes but does not primarily influence osmotic pressure.

d. Potassium: Potassium (K⁺) is the main intracellular cation and affects cell function, but sodium has a more significant role in osmotic pressure regulation.

a. Trachea: The trachea is the windpipe, a tube that carries air from the larynx to the bronchi in the lungs. It does not contain vocal cords.

b. Pharynx: The pharynx is a muscular passageway at the back of the throat that connects the nasal cavity, oral cavity, and larynx. It does not contain vocal cords.

c. Larynx: The larynx, also known as the voice box, is located in the throat and contains the vocal cords. These are two folds of muscle that vibrate when air passes over them, producing sound for speech.

d. Epiglottis: The epiglottis is a small flap of cartilage that covers the opening of the larynx during swallowing. It does not contain vocal cords.