**Core Concept**
The lingual extension of the mandibular impression is a critical aspect of dental impression making, requiring thorough testing to ensure accuracy and prevent errors. The lingual extension should be tested for its proper extension and adaptation to the lingual surface of the mandible, which is essential for creating a precise and comfortable dental prosthesis.

**Why the Correct Answer is Right**
The lingual extension of the mandibular impression should be tested by placing the impression in the patient's mouth and then retracting the tongue to check for proper adaptation to the lingual surface of the mandible. This is done to prevent the impression from being too thin or too thick, which could lead to an ill-fitting dental prosthesis. The lingual extension should be checked for its proper extension, and any areas of thinness or imperfection should be corrected before the impression is sent to the dental laboratory for processing.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify a method for testing the lingual extension of the mandibular impression. Without a clear method, it is impossible to determine if the impression is properly adapted to the lingual surface of the mandible.
**Option B:** This option is incorrect because it may not always be possible or practical to use a mirror to check the lingual extension of the mandibular impression. While a mirror can be used to visualize the lingual surface, it may not provide a clear view of the entire impression, and other methods may be more effective.

**Clinical Pearl / High-Yield Fact**
One high-yield fact to remember is that the lingual extension of the mandibular impression should be at least 3-4 mm thick to ensure proper adaptation to the lingual surface of the mandible and prevent errors in dental impression making.

**Correct Answer: C. This is done to prevent the impression from being too thin or too thick, which could lead to an ill-fitting dental prosthesis.**