## Core Concept
The question pertains to the surveillance and control of infectious diseases, specifically plague. The index mentioned is likely related to a method or indicator used in epidemiology to estimate the potential for an outbreak or explosion of plague cases.

## Why the Correct Answer is Right
The correct answer, **C. Rodent density index**, is related to the estimation of plague outbreaks because rodent populations are often the natural reservoir for the bacteria *Yersinia pestis*, which causes plague. High rodent density can increase the likelihood of transmission to humans, either directly or through fleas. Monitoring rodent density can serve as an early warning system for potential plague outbreaks.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is blank and does not provide any relevant information.
- **Option B:** This option is also blank and does not offer any useful details.
- **Option D:** This option is blank as well, lacking any pertinent information.

## Clinical Pearl / High-Yield Fact
A key point to remember is that plague can manifest in bubonic, pneumonic, and septicemic forms, with different modes of transmission and varying levels of contagiousness. Understanding the ecological and epidemiological factors, such as rodent density, is crucial for public health interventions.

## Correct Answer Line
**Correct Answer: C. Rodent density index**.