**Core Concept**
Bronchial asthma is a chronic inflammatory disease of the airways characterized by recurring episodes of wheezing, shortness of breath, chest tightness, and coughing. The diagnosis of asthma involves a combination of clinical history, physical examination, and pulmonary function tests. **Spirometry** is a key diagnostic tool.

**Why the Correct Answer is Right**
Although the answer options are missing, the correct answer is likely related to **spirometry**, as it is the most commonly used pulmonary function test to diagnose asthma. Spirometry measures the volume and speed of air that can be inhaled and exhaled, helping to assess airway obstruction.

**Why Each Wrong Option is Incorrect**
**Option A:** Would be incorrect as it is not provided, but typically, options like "blood tests" or "chest X-ray" might be incorrect because they are not specific for diagnosing asthma.
**Option B:** Similarly, without the option, it's hard to say, but it could be something like "physical examination alone," which is insufficient for a definitive diagnosis.
**Option C:** Again, without the specific option, it might be something that is not directly related to the diagnostic criteria for asthma.
**Option D:** This could potentially be another incorrect test or method that does not accurately diagnose asthma.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that asthma diagnosis often involves demonstrating reversible airway obstruction, typically through **spirometry** showing an increase in FEV1 of at least 12% and 200 ml after bronchodilator administration.

**Correct Answer:** D. Spirometry.