Final answer:
The Mesopotamians' shift from interpreting omens to studying planetary motion represents the move from astrology to astronomy.
Step-by-step explanation:
The correct answer to the student's question is: C) Mesopotamians; planetary motion. The ancient astronomical observations started in Babylonia, where interpreting omens from the sky was practiced. Over time, their scholars became more focused on detecting and predicting planetary motion rather than identifying omens as astronomical knowledge advanced. Astrology, which originated in Babylonia, was eventually absorbed by the Greeks and developed further. This shift in focus from omens to planetary phenomena represented a significant transition from astrology to astronomy, which is concerned with scientific observations and predictions.