**Core Concept**
Basal metabolic rate (BMR) refers to the energy expended by the body at rest, representing the minimum energy required to maintain vital functions. It is influenced by several factors, including age, sex, weight, and body composition.

**Why the Correct Answer is Right**
The correct answer is **C. Thyroid hormones**. Thyroid hormones, particularly triiodothyronine (T3) and thyroxine (T4), play a crucial role in regulating BMR. They increase the metabolic rate by stimulating cellular respiration and enhancing the activity of enzymes involved in energy production. The thyroid gland produces these hormones, which are then released into the bloodstream and bind to receptors in various tissues, including the liver, muscles, and adipose tissue. This binding process activates the transcription of genes involved in energy metabolism, leading to an increase in BMR.

**Why Each Wrong Option is Incorrect**
**Option A:** Age is an important factor in BMR, but it is not the primary regulator. While BMR does decrease with age, it is not the most significant determinant.
* **Option B:** Sex is also a factor, with females generally having a lower BMR than males due to differences in body composition and hormonal levels. However, sex is not the primary regulator of BMR.
* **Option D:** Weight is a significant factor, as a higher body mass index (BMI) is associated with a higher BMR. However, weight is not the primary regulator of BMR.

**Clinical Pearl / High-Yield Fact**
A key clinical correlation is that hypothyroidism can lead to a decrease in BMR, resulting in weight gain and decreased energy levels. Conversely, hyperthyroidism can cause an increase in BMR, leading to weight loss and increased energy expenditure.

**Correct Answer: C. Thyroid hormones**