A 9-year-old who responded to the original Stanford-Binet with the proficiency of an average 12-year-old was said to have an IQ of

a. 75.

b. 115.

c. 125.

d. 133.

d. 133.

Explanation:

IQ otherwise known as Intelligent Quotient, is a measure of intelligence, which is applied by using ratio of mental age to physical or chronological age, then multiply by 100.

Hence, a child of 9year old had a mental age of 12 year old, (which means, performing on the test meant for an average 12 year old) the child is then assigned IQ of 12/9 * 100 which equals to IQ of 133.

Therefore, a 9-year-old who responded to the original Stanford-Binet with the proficiency of an average 12-year-old was said to have an IQ of 133.