**Core Concept**
Functional residual capacity (FRC) is the volume of air present in the lungs at the end of passive expiration. It represents the balance between the outward recoil of the lungs and the inward recoil of the chest wall. Measuring FRC is crucial in assessing respiratory function, particularly in patients with lung diseases or those requiring mechanical ventilation.

**Why the Correct Answer is Right**
The correct answer is helium dilution technique. This method measures FRC by using a known volume of helium gas, which is inhaled by the patient. The patient then exhales slowly, and the concentration of helium in the exhaled air is measured. By knowing the volume of helium inhaled and the concentration of helium in the exhaled air, FRC can be calculated. This technique is accurate and reliable, making it the best method for measuring FRC.

**Why Each Wrong Option is Incorrect**
**Option A:** Plethysmography is a technique that measures the volume of air in the lungs, but it is not the best method for measuring FRC. It involves sealing the patient in a box and measuring the change in volume of the box as the patient breathes in and out.
**Option B:** Arterial blood gas analysis can provide information about respiratory function, but it does not directly measure FRC.
**Option C:** Spirometry measures lung volumes, but it is not the best method for measuring FRC, especially in patients with airway obstruction.

**Clinical Pearl / High-Yield Fact**
The helium dilution technique for measuring FRC is particularly useful in patients with airway obstruction, as it is less affected by airflow limitation compared to other methods.

**Correct Answer: C. Helium dilution technique.**