Concept:
K. G. Subramanyan was a versatile modern Indian artist.
He worked in many forms such as painting, mural, printmaking, clay, terracotta, design and storytelling.
Step 1: Check Assertion (A).
The assertion correctly states that K. G. Subramanyan was basically a painter and also worked in murals, printmaking, clay and terracotta.
So, Assertion (A) is correct.
Step 2: Check Reason (R).
The Reason says that he wanted to prove himself as a muralist and sculptor.
This is not the correct explanation of his wide artistic practice.
His work in many media came from experimentation, creative exploration and Santiniketan-based artistic thinking.
Step 3: Final conclusion.
Assertion is correct, but Reason is not correct.
Hence:
\[
\boxed{\text{(C)}}
\]