Step 1: Understanding the Concept:
To find the odd one out, look for a pattern in the other options that one option does not share.
Step 2: Detailed Explanation:
1. Content Pattern: Options (B), (C), and (D) describe major epic literary works (\textit{Paradise Lost}, \textit{Ramcharitmanas}, \textit{Mahabharat}) and their legendary authors.
2. Grammar Pattern: Options (B), (C), and (D) are all in the passive voice ("was composed by", "was written by").
3. The Exception: Option (A) describes a simple personal act (writing a letter) and is written in the active voice. It does not mention a monumental literary work in the same way.
Step 3: Final Answer:
Option (A) is the odd one out.