**Core Concept**
Farmer's lung is a type of occupational hypersensitivity pneumonitis caused by the inhalation of antigens from thermophilic actinomycetes, which grow in moldy hay or other agricultural products.

**Why the Correct Answer is Right**
The correct answer involves the pathophysiology of hypersensitivity pneumonitis, a condition characterized by an immune response to inhaled antigens. Thermophilic actinomycetes, such as *Micropolyspora faeni*, *Thermoactinomyces vulgaris*, and *Saccharopolyspora rectivirgula*, are commonly found in moldy hay and other organic materials. When inhaled, these microorganisms trigger an immune response, leading to inflammation and damage in the lungs.

**Why Each Wrong Option is Incorrect**
* **Option A:** This is incorrect because farmer's lung is not caused by an allergic reaction to dust mites, which are more commonly associated with allergic rhinitis and asthma.
* **Option B:** This is incorrect because hypersensitivity pneumonitis is not caused by exposure to chemicals or heavy metals, which can cause different types of lung injury.
* **Option C:** This is incorrect because asbestosis is a type of occupational lung disease caused by exposure to asbestos fibers, not thermophilic actinomycetes.

**Clinical Pearl / High-Yield Fact**
Farmer's lung is a classic example of an occupational lung disease that can be prevented by wearing a mask when handling moldy hay or other agricultural products.

**Correct Answer: D. Thermophilic actinomycetes.**