Answer: D. by sharing a hypothetical story as an example of the subject.
Explanation:
The given excerpt has been taken from The Code Book. The passage makes the readers imagining a scene where Alice wants to send out invitations to her birthday party.
The traditional method of ordinary mail is slow and takes time to reach to the receivers. On the other hand, digital mail is fast and efficient.
Therefore, the correct answer is (D) by sharing a hypothetical story as an example of the subject.