Concept:
This question tests basic astronomical facts and the ability to distinguish between a definition and a specific phenomenon.
Step 1: Evaluating Assertion A.
The Moon is the only natural satellite of Earth, as it orbits our planet. This is a scientific fact. Thus, A is correct.
Step 2: Evaluating Reason R.
A lunar eclipse occurs precisely when the Earth moves between the Sun and the Moon, casting its shadow on the Moon. This is also a scientific fact. Thus, R is correct.
Step 3: Analyzing the Relationship.
While both statements are true, the occurrence of a lunar eclipse (a phenomenon) does not explain *why* the moon is a natural satellite (a definition/status). The Moon is a satellite because it orbits the Earth; the eclipse is just a result of that orbit's alignment. Thus, R is not the correct explanation of A.