According to Wilhelm Stern IQ (Intelligence Quotient) is:
(a) \[{\text{IQ = }}\dfrac{{{\text{Mental age}}}}{{{\text{Actual age}}}}{\text{ }} \times {\text{ 100}}\]
(b) \[\text{IQ = }\dfrac{\text{Actual age}}{\text{Mental age}}\text{ }\times \text{ 100}\]
(c) \[{\text{IQ = }}\dfrac{{{\text{Actual age }} \times {\text{Mental age}}}}{{{\text{100}}}}\]
(d) \[\text{IQ = }\dfrac{\text{ 100}}{\text{Actual age }\times \text{Mental age}}\text{ }\]

Hint: Intelligence quotient is a method of quantitatively estimating intelligence of a person. Before Wilhelm Stern, intelligence was assessed by using certain tests which would find a particular person’s intelligence by comparing it with some standard or by comparing them across a population. Stern suggested use of a mathematical formula which would take a ratio between a person’s mental age and actual age.

Complete answer:
Intelligence quotient is a widely used parameter to determine a person’s mental ability and reasoning ability. It is a standardized test used for assessment of human intelligence. We know the actual age of a person and their mental age can be calculated by making use of certain tests designed for this purpose. The age level of an individual's mental ability is Mental age. It is based on the age of reaching that same level of mental attainment in an average individual. The formula was given by Stern :
\[\text{IQ = }\dfrac{\text{Mental age}}{\text{Actual age}}\text{ }\times \text{ 100}\]
If a person’s mental and actual age match, their IQ will be 100%. If the mental age of a person is more than their actual age, their IQ will be more than 100%. Consequently, if the mental age of a person is less than their actual age, their IQ will be less than 100%. It is observed that people with IQ above or equal to 140 are extremely intelligent.
Hence, correct answer is \[\text{IQ = }\dfrac{\text{Mental age}}{\text{Actual age}}\text{ }\times \text{ 100}\] i.e. option (a).

Note: Even after suggesting a formula that is now used extensively in the world, Stern also suggested not to depend on this value completely i.e. this is not the sole way to categorize one’s intelligence. Individual differences will always be a major factor which influences IQ.