## Core Concept
John Snow's discovery of cholera is a landmark example in the field of epidemiology. It involves the identification of a disease outbreak and the tracing of its source to a specific contaminated water pump. This event showcases the application of epidemiological principles to control a disease outbreak.

## Why the Correct Answer is Right
John Snow's investigation into a cholera outbreak in London in 1854 is a classic example of **epidemiological investigation**. He mapped the outbreak and identified a specific water pump on Broad Street (now Broadwick Street) as the source of the outbreak. By convincing the local authorities to remove the pump, Snow effectively controlled the outbreak. This action demonstrates the power of **descriptive epidemiology** and **interventional epidemiology**, where identifying the source of an outbreak and intervening can prevent further cases.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is not provided, but typically, incorrect options might include types of studies (e.g., randomized controlled trials) or other scientific methods not directly related to epidemiological investigations.
- **Option B:** Similarly, without content, we can infer this might relate to basic science research or laboratory techniques, which, while crucial, do not directly apply to the epidemiological investigation like Snow's.
- **Option C:** This could potentially relate to health policy or public health strategies but is not directly linked to Snow's method of investigating and controlling an outbreak.

## Clinical Pearl / High-Yield Fact
A key takeaway from John Snow's investigation is the importance of **environmental epidemiology** and the role of **waterborne pathogens** in causing disease outbreaks. This historical event underscores the need for clean water sources to prevent the spread of diseases like cholera.

## Correct Answer: D. Epidemiological investigation.