First discovered hormone
A. Thyroxine
B. Adrenaline
C. Secretin
D. Insulin

Hint: Hormones are chemical messengers secreted by various organs of the body directly into the bloodstream, to be sent to the target organs and signal them to initiate their respective functions.

Complete step by step answer:
Hormones are responsible for the chemical regulation of physiological processes. They act as messages from control centers(mainly glands like the pituitary) to the target organs for the initiation of their functions. They can be transmitted via an exocrine (ducts between the source and the target) or an endocrine (secretion into the bloodstream directly) system. Hormones are responsible for the regulation of many different activities of our body like, growth, sex drive, pigmentation, metabolism, cognitive function and mood, reproduction, body temperature etc. The over and under secretion of any hormone can lead to drastic, life altering changes.Analyzing the given options,
1. Thyroxine is also known as the thyroid hormone secreted by the thyroid glands. They are of two types, triiodothyronine \[\left( {{T}_{3}} \right)\]and thyroxine \[\left( {{T}_{4}} \right)\]. Partially composed of iodine, the tyrosine-based hormones are responsible for the regulation of metabolism.
2. Adrenaline is secreted by the adrenal glands. It is called epinephrine when the neuronal cells of the medulla oblongata secrete them to serve as neurotransmitters. Adrenaline is mainly responsible for increasing the heart rate, increasing blood pressure and glucose levels, enlarging the air passage area in lungs, dilation of pupils, etc. It is released while feeling euphoric and excited.
3. Secretin is produced by the S cells of the duodenum and is a peptide hormone. It is responsible for the regulation of homeostasis, regulates secretion in the pancreas, stomach and liver. Secretin was the first ever discovered hormone. It was discovered by E. H. Starling in 1902.
4. Insulin is the main anabolic hormone of the human body. It is secreted by the beta cells of the islets of langerhans in the pancreas. Insulin regulates the homeostasis of glucose in the body.
Hence the correct answer is option C.
Note: Before the discovery of Secretin, physiologists had conflicting views about the nervous system controlling every action. It was known that digestive juices were secreted by the pancreas in response to the passage of food (chyme) through the pyloric sphincter into the duodenum. In the experiment that led to the discovery, the nervous connections to pancreas were barred hence it was established that the process indeed was not regulated by the nervous system.