Aseries of measurements were made on four skulls (descriptions of the measurements can be found in the student exploration sheet). which is most likely a modern human skull?

This question is incomplete. The question gives a choice between four skulls: A, B, C and D. The skulls are compared in a table for: Cranium/face area ratio, face angle and foramen magnum ratio.

The cranium/face area ratio relates to the size of the cranium relative to the rest of the skull, and we would expect this ratio to be fairly large for a human. The face angle relates to human faces in relation to primate faces, which are less protruded forwards (i.e., we have relatively flat faces), and the foramen magnum is the hole in the base of the skull through which the spinal column connects to the skull. In bipeds, we would expect this hole to be fairly equally distant from the back of the skull to the front of the skull, so this ratio would be closer to 1. Based on this information and the ratios given in the table, I would say that skull B is most likely a human skull.

The answer is d. when it begins to fall back to the child's hand