**Core Concept**
The Integrated Child Development Service (ICDS) Scheme is a government initiative aimed at providing essential services to children in the age group of 0-6 years, including nutrition, health, and education. The scheme's success depends on the proportion of children in this age group within the population.

**Why the Correct Answer is Right**
The correct answer is based on the demographic characteristics of India's population. According to the 2011 census, approximately 40% of the population is under the age of 18, and within this group, about 30-40% are in the 0-6 age bracket. This proportion has been relatively stable over the years. The ICDS Scheme targets this vulnerable age group to ensure their proper development and reduce child malnutrition.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it is too low. The proportion of children in the 0-6 age group is significantly higher than this.
**Option B:** This option is incorrect because it is too high. While the proportion of children in this age group is substantial, it is not as high as this option suggests.
**Option C:** This option is incorrect because it is within the plausible range, but the correct answer is more specific and based on the latest demographic data.

**Clinical Pearl / High-Yield Fact**
A key aspect of the ICDS Scheme is its focus on the first 1,000 days of a child's life, which is critical for their development and growth. This period is often referred to as the "window of opportunity" for interventions that can have a lasting impact on a child's health and well-being.

**Correct Answer: D. 30-40%.**