Can anyone provide the explination of the correct answer (please also see the attached file. if that explination makes sense to you)

Question 1:

You are the project manager at a company developing an XYZ car, you have been given project charter, progressively elaborated the requirements and have just completed the Define scope process. However, while you are in Define activities process, you realize that certain deliverable are missing from the define scope process. What is your next plan of action? (correct answer is 4)