Growth does not spread over the body at random; it travels along fixed routes. One route runs lengthwise from the head downwards, so the head and brain mature earliest, then the trunk and arms, and the legs and feet last. A second route runs outwards from the central axis of the body towards the fingers and toes.

Both routes are stated together as the principle that development proceeds in definite directions. This is why an infant can hold up its head before it can sit, sit before it can stand, and control the whole arm before it can grip a crayon with the fingertips. The head to foot route is simply the lengthwise half of this principle.

The other choices describe different regularities. An orderly order of stages says that each stage follows a fixed succession, but it does not name a spatial route along the body. Combining separate movements into one coordinated skill concerns how parts unite into a whole. Since a correct principle is already named, rejecting all choices is unsound.

Principle of directional development option is correct.