**Core Concept**
Smear examination under the RNTCP (Revised National Tuberculosis Control Programme) is a crucial diagnostic tool for detecting Mycobacterium tuberculosis infection. The examination involves counting acid-fast bacilli (AFB) in a sputum sample slide under a microscope with oil immersion, which enhances the visibility of the bacilli.

**Why the Correct Answer is Right**
The correct answer is based on the RNTCP guidelines, which define the criteria for reporting sputum smear examination results. According to these guidelines, a sputum sample is considered positive if 3 or more AFB are seen per oil immersion field. This is a critical threshold for diagnosing pulmonary tuberculosis. The oil immersion technique is used to enhance the visibility of the AFB, which are typically 3-4 microns in size.

**Why Each Wrong Option is Incorrect**
* **Option A:** There is no specific guideline that categorizes a sputum sample with 2 AFB per oil immersion field as positive or negative. It is often considered inadequate for a definitive diagnosis.
* **Option B:** A sputum sample with 4 or more AFB per oil immersion field is considered heavily positive, but the question specifies 3 AFB, which falls into a different category.
* **Option C:** There is no specific guideline that categorizes a sputum sample with 1 AFB per oil immersion field as positive or negative. It is often considered inadequate for a definitive diagnosis.

**Clinical Pearl / High-Yield Fact**
To remember the RNTCP guidelines for reporting sputum smear examination results, the mnemonic "3-4-5" can be used, where 3 AFB per oil immersion field is considered positive, 4 or more is considered heavily positive, and 5 or more is considered a large number of bacilli.

**Correct Answer: C.**