Identify the missing words in the following sentence. The `managing a stage boundary` process should be executed [ ? ] a management stage. 

Answer options:

A. when a stage plan has been prepared for
B. at the beginning of
C. when all the delivery steps have been completed within
D. at, or close to the end of,

D

Reference: https://publications.axelos.com/Prince2Agile2015/content.aspx?showNav=true&expandNav=true&page=cros_131