Concept:
Etching is an intaglio printmaking process.
In etching, acid is used to bite lines into a metal plate.
Step 1: Check Assertion (A).
In etching, acid plays a central role.
The artist draws through a protective ground on the plate.
Then acid bites the exposed lines.
So, Assertion (A) is correct.
Step 2: Check Reason (R).
Etching is commonly done on metal plates such as zinc and copper.
These plates are treated with acid to create lines.
So, Reason (R) is also correct in the context of etching.
Step 3: Check explanation.
The Reason explains why acid is important in etching.
Since the image is created by acid acting on the plate, acid becomes a main part of the process.
Step 4: Final conclusion.
Both Assertion and Reason are correct, and Reason explains Assertion.
Hence:
\[
\boxed{\text{(A)}}
\]