Final answer:
The correct answer is c) 3 (Earth, Venus, Mercury).
Step-by-step explanation:
An Astronomical Unit (AU) serves as a fundamental measurement in astronomy, denoting the mean distance between the Earth and the Sun, approximately 150 million kilometers. In scrutinizing the planetary configurations, it becomes evident that Earth, Venus, and Mercury are the exclusive celestial bodies positioned within this 1 AU threshold from the Sun. This spatial proximity is integral to understanding the inner solar system's dynamics and highlights the distinctive orbital characteristics of these terrestrial planets, distinguishing them from the more distant members of our cosmic neighborhood.