**Core Concept**
The Cheopis index is a measure used to assess the potential for a plague outbreak by quantifying the ratio of the flea vector, Xenopsylla cheopis, to the total number of rodents in a given area. This index is crucial in understanding the risk of bubonic plague transmission.

**Why the Correct Answer is Right**
The Cheopis index is a reliable indicator of potential plague outbreaks because Xenopsylla cheopis is the primary flea vector responsible for transmitting the bubonic plague bacterium, Yersinia pestis, to humans. The higher the ratio of Cheopis fleas to rodents, the greater the likelihood of a plague outbreak. This is because Cheopis fleas are highly efficient in transmitting the disease to rodents and, subsequently, to humans through their bites.

**Why Each Wrong Option is Incorrect**
**Option A:** Total flea index is not a specific indicator of plague transmission, as it includes various flea species that may not be involved in disease transmission.
**Option C:** Specific percentage of fleas is a vague term and does not provide a clear quantitative measure of the risk of plague transmission.
**Option D:** Burrow index is not a relevant indicator for plague transmission, as it refers to the presence of rodent burrows rather than the flea population.

**Clinical Pearl / High-Yield Fact**
The Cheopis index is a valuable tool in epidemiological studies, enabling public health officials to identify areas at high risk of plague outbreaks and implement targeted control measures to prevent the spread of the disease.

**✓ Correct Answer: B. Cheopis index**