Hello Jaydip,
The key to choosing the correct answer is looking for the best answer. If I understand correctly, you want to know whether choice A or B would be the best answer.
Most of the time, the project sponsor should have a good understanding of what key project deliverables are, and naturally we as PMs will approach the sponsor as it is the most direct way. However, the project sponsor may not know every single detail of the project, especially for large and mega projects. Therefore, the best way to find out every project deliverables would be consulting with all the key stakeholders. (The same explanation could be used to explain why D is not the best answer as well).
Hope this helps,
Joe