Concept:
Oleography is a colour printing technique related to lithography.
It was used to reproduce paintings in printed form.
Raja Ravi Varma's press made his mythological images widely available through oleographs.
Step 1: Check Assertion (A).
Ravi Varma Press used oleography to reproduce his artworks.
Oleography is related to lithography.
So, Assertion (A) is correct.
Step 2: Check Reason (R).
Oleography allowed more colourful and painterly reproduction possibilities.
It helped reproduce images in a form that resembled oil paintings.
So, Reason (R) is correct.
Step 3: Check explanation.
The advantage of oleography explains why Ravi Varma Press used it for reproducing artworks.
Thus, Reason correctly explains Assertion.
Step 4: Final conclusion.
Both Assertion and Reason are correct and Reason is the correct explanation.
Hence:
\[
\boxed{\text{(A)}}
\]