Lamarck's idea of "acquired traits" proposed that a. acquired traits are passed down from parent to offspring. b. in animals, traits that are used often will grow and change shape. c. acquired traits are not passed down from parent to offspring. d. in animals, traits that are not used often will shrink and change shape.

The correct answer is-  A. acquired traits are passed down from parent to offspring.

Lamarck proposed a theory of inheritance in 1801, which is called Inheritance of acquired traits/characteristics.

According to this theory, during a life span, a individual brings certain changes in his characteristics in order to adapt to the changing environmental conditions. These changes are called acquired traits and can be passed from one generation to the next generation (that is from parents to their offspring).

Thus, option A) is the right answer.