**Core Concept**
The Developmental Quotient (DQ) is a measure used to assess the cognitive and adaptive abilities of children, particularly those with developmental disabilities. It is calculated based on the ratio of the child's actual developmental age to their chronological age.

**Why the Correct Answer is Right**
The correct formula for calculating the Developmental Quotient is: Average age at attainment / observed age at attainment x100. This ratio provides a standardized score, indicating how well a child's developmental abilities compare to their actual age. The average age at attainment represents the typical age at which a certain developmental milestone is achieved, while the observed age at attainment is the actual age at which the child has achieved that milestone. By dividing the average age by the observed age and multiplying by 100, the DQ score provides a percentage-based measure of the child's developmental progress.

**Why Each Wrong Option is Incorrect**
**Option B:** This option is incorrect because dividing the observed age at attainment by the average age at attainment would result in a score greater than 100, indicating that the child is more advanced than average, which is not the case.

**Option C:** This option is incorrect because multiplying the observed age at attainment by the average age at attainment would result in a score that is not a ratio and would not provide a standardized measure of the child's developmental progress.

**Option D:** This option is incorrect because dividing the observed age at attainment by the average age at attainment would result in a score less than 1, indicating that the child is less advanced than average, which is not the correct interpretation of the DQ score.

**Clinical Pearl / High-Yield Fact**
A DQ score of 70 or below is generally considered to indicate developmental delay or disability, while a score of 80 or above is considered to be within the normal range.

**✓ Correct Answer: A. Average age at attainment / observed age at attainment x100**