**Core Concept**
Proper sanitation and hygiene in public spaces, especially in educational institutions, are crucial for maintaining public health. The World Health Organization (WHO) and other global health authorities provide guidelines for the ideal ratio of toilets to students in schools.

**Why the Correct Answer is Right**
The recommended ratio of toilets to students is based on the need to minimize waiting time, prevent overcrowding, and maintain cleanliness. A commonly cited guideline is to have 1 urinal for every 50-100 boys and 1 latrine for every 25-50 girls. This ratio takes into account the different needs and behaviors of boys and girls, as well as the need for separate facilities for each gender.

**Why Each Wrong Option is Incorrect**
* **Option A:** There is no widely accepted guideline that recommends 1 urinal and 1 latrine per 20 students.
* **Option B:** While some guidelines suggest a higher ratio, 1 urinal and 1 latrine per 200 students may not be sufficient to prevent overcrowding and maintain cleanliness.
* **Option C:** This option is too vague, as the ratio of toilets to students can vary depending on the specific needs and circumstances of the school.

**Clinical Pearl / High-Yield Fact**
The ideal ratio of toilets to students can vary depending on the age and gender of the students, as well as the specific needs of the school. It's essential to consider these factors when planning and designing toilet facilities in educational institutions.

**Correct Answer: D. 100. 50**