**Core Concept**
Farmer's Lung is a type of occupational hypersensitivity pneumonitis caused by inhalation of thermophilic actinomycetes, such as *Micropolyspora faeni*, which are commonly found in moldy hay or grain. This condition is a classic example of an allergic reaction to inhaled antigens, leading to an immune-mediated response in the lungs.

**Why the Correct Answer is Right**
The correct answer, *Micropolyspora faeni*, is a thermophilic bacterium that grows in moldy hay or grain, producing antigens that trigger an allergic response in susceptible individuals. When inhaled, these antigens stimulate an immune response, leading to the release of various cytokines and chemokines, which recruit inflammatory cells to the lungs, causing symptoms of hypersensitivity pneumonitis. The exact mechanisms involve the recognition of bacterial antigens by T-cells and the activation of macrophages and other immune cells.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not mention a specific type of bacterium or antigen associated with Farmer's Lung. While various types of dusts and antigens can cause hypersensitivity pneumonitis, *Micropolyspora faeni* is the most commonly implicated agent in Farmer's Lung.
* **Option B:** This option is incorrect because it refers to a different type of occupational lung disease, Asbestosis, which is caused by inhalation of asbestos fibers, not thermophilic actinomycetes.

**Clinical Pearl / High-Yield Fact**
Farmer's Lung is often referred to as a "moldy hay disease" and is a classic example of an occupational lung disease caused by inhalation of specific antigens. It is essential to recognize the clinical presentation of Farmer's Lung, which includes symptoms such as cough, dyspnea, and chest tightness, and to consider the occupation and environmental exposure history of the patient.

**Correct Answer: A. Micropolyspora faeni**