**Core Concept**
Incidence of diarrhoea in a community can be calculated using epidemiological methods, specifically by determining the rate at which new cases of diarrhoea occur within a defined population over a specified period. This involves calculating the number of new cases and dividing it by the population at risk.

**Why the Correct Answer is Right**
The correct method for calculating the incidence of diarrhoea is by using the formula: Incidence Rate = Number of new cases / Population at risk x Time. This formula is essential in surveillance and outbreak investigations to understand the burden of diarrheal diseases in a community. For instance, a diarrhoea outbreak may be declared when the incidence rate exceeds a certain threshold, indicating a need for public health intervention.

**Why Each Wrong Option is Incorrect**
**Option A:** Incidence cannot be calculated by simply dividing the number of diarrhoea cases by the population. This would result in a prevalence rate, which is a measure of the total number of cases present in the population at a given time.
**Option B:** The formula for calculating the incidence rate involves a time component, which is not present in this option.
**Option C:** This option is incomplete and does not provide a clear method for calculating the incidence of diarrhoea.

**Clinical Pearl / High-Yield Fact**
When conducting epidemiological investigations, it's essential to differentiate between incidence and prevalence rates to accurately describe the burden of diarrheal diseases in a community.

**Correct Answer: C. (Number of new cases / Population at risk x Time)**