**Core Concept**
The Sade's classification is a grading system used to describe the mobility of the tympanic membrane. It ranges from grade 0 to grade 4, where grade 0 represents a tympanic membrane that is fixed to the promontory and grade 4 represents normal mobility. The classification helps in assessing the severity of middle ear pathology, particularly in the context of otosclerosis and ossicular chain fixation.

**Why the Correct Answer is Right**
In Sade's classification, a tympanic membrane that is retracted and touches the promontory is considered to be of grade 0. This indicates a significant loss of mobility, often due to ossicular chain fixation or otosclerosis. The tympanic membrane's inability to move freely in response to sound waves is a hallmark of this condition. In such cases, the tympanic membrane appears to be in close contact with the promontory, indicating a loss of the normal air space between them.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because a tympanic membrane that is retracted but not touching the promontory would be classified as grade 1 in Sade's classification.

**Option B:** This option is incorrect because a tympanic membrane that is mobile but shows slight retraction would be classified as grade 2 in Sade's classification.

**Option D:** This option is incorrect because a tympanic membrane that is mobile and shows no retraction would be classified as grade 4 in Sade's classification, indicating normal mobility.

**Clinical Pearl / High-Yield Fact**
In clinical practice, it is essential to remember that Sade's classification is a useful tool for assessing the severity of middle ear pathology. A grade 0 classification, where the tympanic membrane is fixed to the promontory, often indicates a need for further investigation and management, such as surgical intervention.

**Correct Answer:** C.