D. an introductory step toward a goal.
In the context of the paragraph, the word "initiative" is used to describe the reporter's first step or approach to his assignment, which was to visit Gatsby's house in order to gather information for his story. This suggests that "initiative" refers to an introductory step toward a goal, rather than an attempt to solve a problem, an intense curiosity, or the ability to judge what needs to be done and then act.