**Core Concept**
Hookworm infestation is a significant public health concern due to its impact on nutritional status, particularly iron deficiency anemia, in endemic regions. The Chandler's index is a diagnostic tool used to assess the intensity of hookworm infestation. It is calculated by dividing the number of eggs per gram of stool by the number of eggs per gram of feces in a control sample.

**Why the Correct Answer is Right**
The Chandler's index is a measure of hookworm egg intensity in stool samples. An index of 0.01 or more is considered a major public health problem, indicating a high intensity of infestation. This threshold is based on the observation that hookworm infestation leads to significant nutritional deficiencies and anemia in individuals with egg intensities above this level. The presence of hookworm eggs in stool is indicative of active infection, and the Chandler's index helps in assessing the severity of the infestation.

**Why Each Wrong Option is Incorrect**
**Option A:** An index of 0.001 is considered a low intensity of infestation and is not a major public health problem. Individuals with this level of infestation are less likely to experience significant nutritional deficiencies.

**Option B:** An index of 0.1 is considered a moderate intensity of infestation, but it is not a major public health problem. Individuals with this level of infestation may experience mild nutritional deficiencies, but it is not severe enough to warrant a major public health response.

**Option D:** An index of 0.0001 is considered a very low intensity of infestation and is not a major public health problem. Individuals with this level of infestation are unlikely to experience any significant nutritional deficiencies.

**Clinical Pearl / High-Yield Fact**
The Chandler's index is a useful tool for assessing the intensity of hookworm infestation, but it has limitations. Other factors, such as the presence of other parasites or gastrointestinal diseases, can also affect the test results. Therefore, it is essential to consider the clinical context and other diagnostic tests when interpreting the Chandler's index.

**Correct Answer:** C. 0.01 or more.