**Core Concept**
The question is testing the student's knowledge about plaque indices, which are used to assess the accumulation of dental plaque, a biofilm of bacteria that can lead to oral diseases. Plaque indices often involve visual examination of the teeth and can be categorized into different types based on their methodology.

**Why the Correct Answer is Right**
The correct answer is likely the one that does not require the use of a disclosing agent, which is a dye that stains the plaque allowing for its visualization. Some plaque indices, like the Quigley-Hein plaque index, use a disclosing agent to facilitate the assessment of plaque accumulation. However, the Silness-Loe plaque index is notable for not using a disclosing agent, relying instead on the direct observation of plaque by the examiner. This index assesses the presence and severity of plaque on the gingival margin and the interdental areas.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the Quigley-Hein plaque index, which uses a disclosing agent, is a well-established method for assessing plaque accumulation.
**Option B:** This option is incorrect because the Turesky-Gilmore-Glickman modification of the Quigley-Hein plaque index also employs a disclosing agent.
**Option C:** This option is incorrect because the Silness-Loe plaque index is indeed known for not using a disclosing agent, making it distinct from the other options.

**Clinical Pearl / High-Yield Fact**
The Silness-Loe plaque index emphasizes the importance of direct observation in assessing plaque accumulation, highlighting the need for thorough visual examination in clinical practice.

**Correct Answer:** C. Silness-Loe plaque index.