**Core Concept**
The question is testing the understanding of the mode of transmission of infectious diseases, specifically the concept of waterborne transmission. Waterborne transmission occurs when pathogens are present in contaminated water, which can lead to outbreaks of waterborne diseases.

**Why the Correct Answer is Right**
The correct answer is related to the mode of transmission of infectious diseases, specifically waterborne transmission. In this scenario, a well of contaminated water leads to an epidemic of acute watery diarrhea. This is a classic example of waterborne transmission, where the pathogen (likely a bacterium or virus) is present in the water and causes disease when ingested. The pathogens can multiply in the water, leading to a large number of cases.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not relevant to the scenario described. While foodborne transmission is a common mode of transmission, it is not related to contaminated water.
**Option B:** This option is also not relevant to the scenario. Airborne transmission typically involves respiratory pathogens, which are not relevant to acute watery diarrhea.
**Option C:** This option is incorrect because vectorborne transmission involves the bite of an infected insect, which is not relevant to contaminated water.

**Clinical Pearl / High-Yield Fact**
Waterborne transmission is a common mode of transmission for several pathogens, including Vibrio cholerae, which causes cholera, and Giardia lamblia, which causes giardiasis. Both of these pathogens can cause acute watery diarrhea.

**Correct Answer:** B. Waterborne transmission