**Core Concept**
The physical law used in flow meters is based on the principle of measuring the velocity or volume of fluid flow. This is crucial in various medical applications, such as monitoring blood flow in patients with cardiovascular disease or measuring the infusion rate of intravenous medications.

**Why the Correct Answer is Right**
The correct answer is based on the application of Bernoulli's principle, which states that an increase in the speed of a fluid occurs simultaneously with a decrease in pressure or a decrease in the fluid's potential energy. In flow meters, this principle is utilized to measure the velocity of fluid flow. By applying Bernoulli's equation, the flow rate can be calculated, allowing for accurate measurement and monitoring of fluid flow.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not relate to the physical law used in flow meters. The law of conservation of energy is a fundamental principle in physics but is not directly applied in flow meter measurements.
**Option B:** This option is incorrect because it refers to a principle used in thermodynamics, not fluid dynamics. The ideal gas law is essential in calculating the properties of gases, but it is not relevant to flow meter measurements.
**Option C:** This option is incorrect because it refers to a principle used in quantum mechanics, not fluid dynamics. The Heisenberg uncertainty principle is a fundamental concept in quantum physics, but it is not applied in flow meter measurements.

**Clinical Pearl / High-Yield Fact**
In clinical settings, accurate measurement of fluid flow is crucial in managing patients with cardiovascular disease, sepsis, or those requiring fluid resuscitation. A common clinical trap is mistaking the flow rate for the volume of fluid infused, which can lead to incorrect dosing of medications or mismanagement of fluid status.

**Correct Answer: C. Bernoulli's principle.