**Core Concept**
The correct answer involves understanding the concept of adult learning principles, specifically the importance of hands-on experience and demonstration in teaching medical knowledge to healthcare workers in low-resource settings. This is in line with the principles of the "Teach Back" method, where learners are encouraged to demonstrate their understanding by teaching the information back to the educator.

**Why the Correct Answer is Right**
The correct method of teaching about Oral Rehydration Solution (ORS) to urban stum (a common term for a community health worker in some regions) involves a hands-on approach, where the educator demonstrates the proper mixing of ORS and shows the learner how to administer it to a patient. This approach allows the learner to directly observe and participate in the process, increasing their understanding and retention of the material. By teaching the learner how to teach others, the educator is promoting a cascade effect of knowledge, where the learner becomes a resource for their community. This method also acknowledges the learner's prior knowledge and experience, adapting the teaching to their needs.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because simply lecturing the learner about ORS without any hands-on experience is unlikely to result in long-term retention of the information. Lecture-based teaching methods are often ineffective in adult learners, especially in low-resource settings where practical skills are essential.
**Option B:** This option is incorrect because relying solely on written materials, such as pamphlets or posters, may not be sufficient to teach a complex topic like ORS administration. While written materials can be a useful supplement, they should not be the primary method of teaching.
**Option C:** This option is incorrect because simply observing the educator demonstrate ORS administration is not enough to ensure the learner understands the material. The learner needs to be actively engaged and participating in the process to retain the information.

**Clinical Pearl / High-Yield Fact**
A key principle of effective teaching in low-resource settings is to use a "train-the-trainer" approach, where educators train community health workers to teach others in their community. This approach promotes a cascade effect of knowledge, where the educator trains the learner, who then trains others, increasing the reach and impact of the education.

**Correct Answer: D. Demonstration and practice of ORS administration.**