**Core Concept**
The patient's asthma diagnosis requires a pulmonary function test that measures airflow limitation and reversibility. Spirometry, a non-invasive test, is the primary tool for assessing lung function in asthma patients, evaluating the severity of symptoms, and monitoring treatment effectiveness.

**Why the Correct Answer is Right**
Spirometry measures the volume and flow of air in and out of the lungs. In asthma, airway inflammation and hyperresponsiveness lead to bronchoconstriction, which reduces airflow. The test assesses forced expiratory volume in one second (FEV1) and forced vital capacity (FVC). In asthma, FEV1/FVC ratio decreases due to airway obstruction. After bronchodilator administration, a significant increase in FEV1 indicates reversibility, which is a hallmark of asthma.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not relevant to pulmonary function tests for asthma diagnosis.
* **Option B:** This option might be relevant for other respiratory conditions, but not specifically for asthma diagnosis.
* **Option C:** This option is incorrect because it does not specifically address the need for spirometry in asthma diagnosis.

**Clinical Pearl / High-Yield Fact**
In asthma, a 12% or greater increase in FEV1 following bronchodilator administration is considered a positive response, indicating reversibility and supporting a diagnosis of asthma.

**Correct Answer: C. Spirometry**