Which of the following is a primary law of physical development?

Concept: Child-development pedagogy identifies several general laws of development; one law specifically describes the pace at which physical growth proceeds — growth is not uniform across childhood. It alternates between phases of rapid growth and phases of slower, steady growth.

Application: From birth to about age 2, physical growth — height, weight, and the size of most organs other than lymphoid tissue — is rapid. From roughly age 2 until adolescence, growth continues at a slower, fairly steady annual rate, before accelerating again at puberty. This alternating rapid-then-steady pattern over time is exactly what the 'Law of Rapid Growth' names, making it the primary law of physical development among the given options.

Cross-check — why the other options are not laws of physical development:

• Law of difference from mental development contrasts physical development against mental development; it does not describe how physical growth itself paces over time.

• Law of irregular development refers to different body organs and systems maturing at different, uneven rates relative to one another (for example, the head and brain reaching adult size well before the reproductive organs) — a distinct idea from the time-based rapid/slow growth pattern tested here.

• Law of relation from imagination and emotional development links physical growth to imagination and emotional development; it is a cross-domain relational claim, not a description of the physical growth-rate pattern itself.