Which of the following pairs of words are synonymous?

The correct pair of synonymous words is comprised of "micturition" and "urination." Both terms refer to the process of discharging urine from the urinary bladder through the urethra. This term is often used in medical contexts and serves as a more technical way to describe the same physiological process that the common term "urination" describes.

Other pairs provided, while related to bodily functions, do not share the same meaning. For instance, defecation refers to the elimination of solid waste from the digestive tract, while urination pertains specifically to liquid waste. Respiration includes a broader process of gas exchange, while inhalation specifically refers to the act of taking air into the lungs. Lastly, perspiration involves the secretion of sweat, while absorption refers to the process of taking in nutrients or substances, often through the skin or digestive system. Thus, "micturition" and "urination" are the only pair that truly convey the same concept.