## Core Concept
The question assesses understanding of developmental delay and its quantification. **Developmental quotient (DQ)** is a measure used to assess the developmental age of a child relative to their chronological age, similar to how intelligence quotient (IQ) is calculated. It is calculated as: DQ = (Developmental Age / Chronological Age) * 100.

## Why the Correct Answer is Right
Given that the child has a developmental age of 1 year and a chronological age of 5 years, we can calculate the developmental quotient (DQ) as follows: DQ = (1 / 5) * 100 = 20. This indicates severe developmental delay. The correct answer corresponds to a DQ value of 20.

## Why Each Wrong Option is Incorrect
- **Option A:** If the DQ were higher (for example, 50), it would indicate a less severe delay, which does not match the given developmental and chronological ages.
- **Option B:** Similarly, a higher DQ value would not accurately reflect the significant delay observed in this child.
- **Option D:** A lower DQ value might indicate an even more severe delay than observed, which does not align with the provided ages.

## Clinical Pearl / High-Yield Fact
A significant developmental delay is indicated by a DQ below 70. Causes can be genetic, environmental, or a combination of both. Early identification and intervention are crucial for helping children catch up with their peers.

## Correct Answer Line
**Correct Answer: C. 20.**