The correct answer is option 3: "controls metabolic rate."

The thyroid gland is responsible for producing various hormones, one of which is called "Thyroxine" or T4. Thyroxine plays a crucial role in regulating the metabolic rate of the body. Metabolic rate refers to the rate at which the body converts food into energy and performs various chemical reactions necessary for normal bodily functions.

Thyroid hormones, including thyroxine, help control the metabolic rate by influencing the function of almost every organ system in the body. They affect how quickly the body burns calories, how efficiently it uses energy, and how it regulates body temperature.

An underactive thyroid (hypothyroidism) can lead to a slower metabolic rate, causing symptoms such as weight gain, fatigue, and feeling cold. On the other hand, an overactive thyroid (hyperthyroidism) can result in an increased metabolic rate, leading to symptoms like weight loss, increased heart rate, and feeling hot.

In conclusion, thyroxine, produced by the thyroid gland, is responsible for controlling the metabolic rate of the body.