**Core Concept**
Physiological uncouplers are substances that disrupt the link between ATP synthesis and the electron transport chain in mitochondria, thereby increasing heat production and energy expenditure. This process is crucial for regulating body temperature, particularly in response to cold exposure.

**Why the Correct Answer is Right**
Thyroxine, a hormone produced by the thyroid gland, acts as an uncoupler by increasing the permeability of the inner mitochondrial membrane to protons, thereby dissipating the proton gradient and preventing ATP synthesis. Free fatty acids, released from adipose tissue during fasting or cold stress, also function as uncouplers by binding to the mitochondrial inner membrane and disrupting the proton gradient. Thermogenin, also known as uncoupling protein 1 (UCP1), is a protein expressed in brown adipose tissue that directly uncouples the electron transport chain from ATP synthesis, generating heat instead. The combined action of these substances allows the body to increase its energy expenditure and generate heat in response to cold exposure.

**Why Each Wrong Option is Incorrect**
**Option A:** Thyroxine is indeed an uncoupler, but the question asks for a physiological uncoupler, which implies a substance that is endogenously produced or present in the body. Thyroxine is a hormone, but it is not the only option, so this option is incorrect.
**Option B:** Free fatty acids are physiological uncouplers, but the question asks for all the correct options. This option is incorrect because it does not include other correct options.
**Option C:** Thermogenin is a physiological uncoupler, but the question asks for all the correct options. This option is incorrect because it does not include other correct options.

**Clinical Pearl / High-Yield Fact**
Thermogenin is highly expressed in brown adipose tissue, which is highly active in neonates and adults exposed to cold stress. This tissue is highly metabolically active and generates heat through non-shivering thermogenesis, which is essential for maintaining body temperature in cold environments.

**✓ Correct Answer: D. All of the above**