Who devised the concept of Intelligence Quotient (IQ)?

The correct option is William Stern.

To understand why William Stern is the correct answer, let us look at the history and development of intelligence testing step-by-step:

1. Binet-Simon Scale and Mental Age:
In 1905, French psychologists Alfred Binet and Theodore Simon created the first practical intelligence test to help identify school children who required special instruction. Binet introduced the concept of Mental Age (MA), which represents a child's psychological development relative to the average development of children of the same chronological age.

2. William Stern's Formulation of IQ:
In 1912, German psychologist William Stern recognized that comparing mental age directly to chronological age by subtraction was not consistent across different developmental stages. To resolve this, he proposed dividing the mental age by the chronological age to establish a relative ratio. Stern named this ratio the Intelligence Quotient (IQ). The original ratio devised by Stern is represented as:

$$\mathrm{IQ}=\frac{\text{Mental Age (MA)}}{\text{Chronological Age (CA)}}$$

3. Lewis Terman's Adaptation:
In 1916, Lewis Terman of Stanford University revised the Binet-Simon test to create the Stanford-Binet Intelligence Scale. He adopted Stern's ratio and multiplied it by 100 to eliminate the decimal points, leading to the well-known formula:

$$\mathrm{IQ}=\frac{\text{Mental Age (MA)}}{\text{Chronological Age (CA)}}\times 100$$

Because William Stern was the psychologist who originally formulated the ratio and coined the term "Intelligence Quotient," he is credited with devising the concept.