Concept:
Buddhist art changed significantly with the development of different Buddhist traditions.
Early Buddhist art often used symbols to represent Buddha.
Later, the human image of Buddha became popular.
Step 1: Check Assertion (A).
The assertion says that the division between Hinayana and Mahayana is important for artistic development.
This is correct because Buddhist art changed with doctrinal and devotional developments.
Step 2: Check Reason (R).
The reason says that the representation of Buddha in human form came with the advent of Mahayana sect.
This is also correct in the context of Buddhist art history.
Mahayana Buddhism encouraged devotional worship and image-making.
Step 3: Check explanation.
The reason explains why the division is important for art.
The shift from symbolic representation to human image of Buddha was a major artistic development.
Step 4: Final conclusion.
Both Assertion and Reason are correct, and Reason correctly explains Assertion.
Hence:
\[
\boxed{\text{(A)}}
\]