(A) Pituitary
The pituitary gland is a single (unpaired) gland located at the base of the brain, whereas ovaries, testes, and adrenal glands occur as pairs in the human body.
The question tests knowledge of paired vs. unpaired glands. Ovaries (female), testes (male), and adrenal glands all occur as pairs. The pituitary is a single master endocrine gland — examiners expect you to identify it as the odd one out. Remember: "pair" means two of the same gland exist in the body.