Concept:R.M. Gagne, an American educational psychologist, wrote 'The Conditions of Learning' and proposed a hierarchy of learning based on complexity.Explanation:Gagne's book organizes learning tasks into a hierarchy with eight intellectual levels and nine events of instruction.This model shows that different levels of learning require different types of instruction.A learner must first master lower intellectual levels before moving to higher ones.Answer:D. R.M. Gagne