Orthogenetic (differentiation) refers to:

• A. Simple and complex; growth of fetus and growth of abilities
• B. Change is qualitative and step-like
• C. Growth is passive to environment
• D. Adolescence is the primary period

Answer: (A)

Orthogenetic differentiation is the idea that development starts in a simple, undifferentiated state and becomes progressively more differentiated and complex. Early in life, growth is broad and general, but over time structures, functions, and behaviors specialize and integrate to form more complex abilities. This pattern is evident from fetal development—where simple tissues differentiate into organs—and from later growth, where basic actions and skills become more refined and specialized. That’s why describing development as moving from simple to complex, spanning from the fetus to developed abilities, best captures this concept.

The other options don’t fit as well. Conceptually, change isn’t primarily about being step-like or purely passive to the environment, and adolescence isn’t the central period for this pattern.