Concept:
Ashokan pillars are important examples of Mauryan art.
The Sarnath Lion Capital is especially famous and is considered one of the finest examples of Mauryan sculpture.
Step 1: Check Assertion (A).
The assertion says that the Sarnath Pillar is considered the greatest Ashokan pillar.
This is correct because the Sarnath pillar and its lion capital are highly admired for artistic quality, symbolism, and craftsmanship.
Step 2: Check Reason (R).
The reason says that all parts of Sarnath Pillar are ideal examples of stone pillar.
This is also correct.
The pillar, capital, animals, wheel, and polish reflect excellent stone craftsmanship.
Step 3: Check explanation.
The excellence of all parts of the pillar explains why it is considered one of the greatest Ashokan pillars.
Thus, Reason correctly explains Assertion.
Step 4: Final conclusion.
Both statements are correct and Reason is the correct explanation of Assertion.
Hence:
\[
\boxed{\text{(A)}}
\]