Concept:
The Basohli School of Painting flourished in the late 17\textsuperscript{th} century in the Himalayan region. It is known for its bold colours, expressive faces, and devotional as well as literary themes.
One of its most celebrated illustrated series is the {Rasamanjari of Bhanu Datta, which became the most popular subject among Basohli painters. Besides literary themes, Basohli artists also produced portraits of local rulers, queens, princes, and courtiers.
Step 1: Examine Assertion (A).
The Rasamanjari of Bhanu Datta is indeed the most celebrated and widely illustrated theme in Basohli painting.
\[
\boxed{\text{Assertion (A) is Correct}}
\]
Step 2: Examine Reason (R).
Basohli artists also painted portraits of local kings, their consorts, and courtiers.
Thus,
\[
\boxed{\text{Reason (R) is also Correct}}
\]
Step 3: Check whether (R) explains (A).
Although portrait painting was practiced, it does not explain why the Rasamanjari became the most popular theme.
Therefore,
\[
\boxed{\text{Both (A) and (R) are correct, but (R) is not the correct explanation of (A).}}
\]
Hence,
\[
\boxed{\text{(B)}}
\]