Answer:
He was happy with the few writing samples I’d brought to the interview and offered me the job on the spot.
Step-by-step explanation:
The sentence from the excerpt that most effectively develops the idea that the narrator is a competent writer is "He was happy with the few writing samples I’d brought to the interview and offered me the job on the spot" because this sentence expresses that the editor of the newspaper liked the samples the narrator showed him and that he offered the job means that he thought that the narrator is a good writer.