**Core Concept**
Farmer's Lung is a type of hypersensitivity pneumonitis, a lung disease caused by repeated inhalation of specific antigens. This condition is characterized by an immune response to inhaled particles, leading to inflammation and damage in the lungs.

**Why the Correct Answer is Right**
The causative agent of Farmer's Lung is thermophilic actinomycetes, particularly *Micropolyspora faeni* and *Thermophilic actinomycetes*. These microorganisms are commonly found in moldy hay and other agricultural products. When inhaled, they trigger an immune response, leading to the release of cytokines and the activation of macrophages, which ultimately cause inflammation and damage in the lungs. The pathogenesis of Farmer's Lung involves the deposition of immune complexes in the lung interstitium, leading to an immune-mediated response.

**Why Each Wrong Option is Incorrect**
**Option A:** *Aspergillus fumigatus* is a causative agent of allergic bronchopulmonary aspergillosis (ABPA), a different type of hypersensitivity reaction. It is not associated with Farmer's Lung.
**Option B:** *Histoplasma capsulatum* is a causative agent of histoplasmosis, a fungal infection that primarily affects the lungs. It is not related to Farmer's Lung.
**Option C:** *Mycobacterium tuberculosis* is the causative agent of tuberculosis, a bacterial infection that primarily affects the lungs. It is not associated with Farmer's Lung.

**Clinical Pearl / High-Yield Fact**
Farmer's Lung is a classic example of an occupational lung disease, highlighting the importance of proper ventilation and protective equipment in agricultural settings.

**Correct Answer:** C. Thermophilic actinomycetes.