First off, I think a bit of vocab might help; "Instantiate" roughly means "representation of an idea or abstraction by a specific instance." A penguin instantiates the concept of 'aquatic bird,' for example. Similarly, the MCAT instantiates the category of 'standardized test,' the activity of 'pre-med hazing,' and quality of 'life-ruining timesuck.' I don't think you need to know that definition to get the question right, but knowing what the word means helps make sense of the 3rd and 4th sentences of paragraph 2.
The question is not asking about the 1st person schema-judgment that is described in the paragraph, but rather what a third person would need to do to determine whether or not a particular instantiation has occurred. But in order to know whether or not something has occurred, a third-person researcher first needs to know whether or not it could occur. Since the second paragraph describes the judgment criteria for schema selection, and notes that the information used for selection has to be "similar enough to the context of the schema" in order to instantiation to occur, it's permissible to infer that information dissimilar to the context of the schema will likely prevent a schema's use. Choice D is thus the only answer that could be necessary for any instantiation determination from a third-person perspective.
I'm not sure whether choice B is wrong or not; I haven't taken more than a semester of psychology and don't have a robust understanding of schematics. Unfortunately this is one of those questions where choice D is so much more right than any other choice that they just expect you to be able to select it. Hope that helps.