Concept:
Pyramidal composition is a triangular arrangement of figures or forms.
It gives balance, stability and harmony to a painting.
It was strongly used in Renaissance painting.
Step 1: Check Assertion (A).
Renaissance artists used pyramidal composition in many paintings.
Mannerist artists later moved towards elongated figures, artificial poses and more complex compositions.
So, Assertion (A) is correct.
Step 2: Check Reason (R).
The Reason says that pyramidal composition was revived in early nineteenth-century French Romantic painting.
This is also correct because Romantic compositions often revived dramatic triangular or pyramidal arrangements.
Step 3: Check explanation.
Although both statements are correct, the revival of pyramidal structure in French Romantic painting does not explain why Mannerist artists did not follow Renaissance pyramidal composition.
Therefore, Reason is not the correct explanation of Assertion.
Step 4: Final conclusion.
Both statements are correct, but Reason is not the correct explanation.
Hence:
\[
\boxed{\text{(B)}}
\]