**Core Concept**
Physiological uncouplers are molecules that disrupt the coupling between the electron transport chain and ATP synthesis in mitochondria, leading to an increase in metabolic rate and heat production. This process is mediated by the mitochondrial membrane potential and is often seen in brown adipose tissue, where it helps to generate heat for thermogenesis.

**Why the Correct Answer is Right**
The correct physiological uncoupler among the options is **2,4-Dinitrophenol (DNP)**. DNP works by increasing the permeability of the mitochondrial inner membrane to protons, thereby dissipating the proton gradient and preventing the synthesis of ATP. This leads to an increase in metabolic rate and heat production, making DNP a potent uncoupler of oxidative phosphorylation.

**Why Each Wrong Option is Incorrect**
**Option A:** **Ascorbic acid** is an antioxidant that can donate electrons to the electron transport chain, but it is not a physiological uncoupler. It helps protect cells from oxidative damage by scavenging free radicals.

**Option B:** **Doxorubicin** is an anthracycline antibiotic that can inhibit mitochondrial function, but it is not a physiological uncoupler. It works by intercalating into DNA and inhibiting topoisomerase II, leading to cell death.

**Option C:** **Rotenone** is an insecticide that can inhibit Complex I of the electron transport chain, but it is not a physiological uncoupler. It works by binding to the iron-sulfur centers of Complex I, leading to a decrease in electron transport and ATP synthesis.

**Clinical Pearl / High-Yield Fact**
Physiological uncouplers like DNP are often used in research to study the mechanisms of thermogenesis and to develop new treatments for obesity and metabolic disorders. However, they can also have toxic effects on the body, including hyperthermia and metabolic acidosis.

**Correct Answer:** D. 2,4-Dinitrophenol (DNP)