We first look for a subject that can introduce the sentence meaningfully. Sentence C — "the writer can only be fertile if" — works best as an opener because it clearly sets a condition for the writer’s creativity.
Following C, sentence B — "he renews himself and" — continues the condition by describing the first requirement for the writer’s fertility, maintaining grammatical flow.
Next, sentence A — "he can only renew himself if his soul" — expands on the renewal process by specifying what is needed for it to happen.
Finally, sentence D — "is constantly enriched by fresh experience" — completes the thought, explaining that the soul’s enrichment through experience is the foundation for renewal and, consequently, for a writer’s productivity.
The complete coherent sentence is:
C → B → A → D, matching option (a) CBAD.