**Core Concept**
Oral Rehydration Solution (ORS) is a life-saving intervention for dehydration due to diarrhoea, especially in resource-limited settings like urban slums. Effective teaching in such populations requires a method that combines clarity, visual engagement, and hands-on learning to ensure comprehension and immediate application.

**Why the Correct Answer is Right**
Demonstration is the most effective method in low-literacy, high-mobility urban slum populations. It allows learners to see the step-by-step preparation of ORS using simple, accessible materials, reinforcing understanding through real-time visual and physical interaction. This method aligns with the principles of **sensory learning** and **active participation**, which are crucial in environments where literacy levels are low and trust in health messages is variable. Demonstrations also enable immediate feedback and correction, increasing retention and practical competence.

**Why Each Wrong Option is Incorrect**
Option A: Lecture is ineffective in slum settings due to low literacy and poor attention spans; passive listening does not ensure retention or practical application.
Option B: Role play, while valuable in clinical training, is impractical in slum outreach settings where participants may not have access to safe environments or trained actors.
Option D: Flash cards are visually simple but lack interactive engagement and fail to teach the practical preparation of ORS, which is essential for real-world use.

**Clinical Pearl / High-Yield Fact**
In diarrhoea management, ORS should be prepared by dissolving 1 liter of clean water with 6 teaspoons of sugar and 1/2 teaspoon of salt — a ratio that must be taught clearly and practically. Demonstrations ensure that even illiterate individuals can follow and replicate the steps correctly.

✓ Correct Answer: C. Demonstration