**Core Concept**
Otosclerosis is a primary bone disorder of the middle ear characterized by abnormal bone growth in the middle ear, leading to fixation of the stapes bone and conductive hearing loss. The process involves abnormal bone remodeling, with a predominance of woven bone formation, which is resistant to osteoclastic resorption.

**Why the Correct Answer is Right**
The most common site involved in otosclerosis is the stapes footplate, specifically the oval window niche. This is due to the unique anatomy of the stapes footplate, which is a thin, bony structure that is susceptible to abnormal bone growth. The oval window niche is a critical site for sound transmission from the inner ear to the middle ear, and fixation of the stapes footplate at this site leads to conductive hearing loss. The process of otosclerosis involves an abnormal increase in the expression of bone morphogenetic protein (BMP) and the activation of osteoblasts, leading to the formation of woven bone.

**Why Each Wrong Option is Incorrect**
**Option A:** The cochlea is not typically involved in otosclerosis, as the process primarily affects the middle ear rather than the inner ear.
**Option B:** The sigmoid sinus is a venous structure located in the posterior fossa, and is not a site of abnormal bone growth in otosclerosis.
**Option C:** The Eustachian tube is a mucosal structure that connects the middle ear to the nasopharynx, and is not typically involved in the process of otosclerosis.

**Clinical Pearl / High-Yield Fact**
Otosclerosis is more common in women than men, and is often associated with a family history of the condition. It is also more common in individuals with a history of autoimmune disorders, such as rheumatoid arthritis.

**Correct Answer: C. The oval window niche.**