Choice D is the best answer. The first paragraph of the passage introduces research by Harvard psychology professor Daniel Wegner demonstrating that the Internet is changing "the way our memories function." One finding of Wegner's study, as stated in the second paragraph, is that "when people have access to search engines, they remember fewer facts and less information because they know they can rely on 'search' as a readily available shortcut." In the third paragraph, Wegner claims that his study shows how "the Internet has become part of a transactive memory source, a method by which [humans'] brains compartmentalize information," such that "computers and technology as well are becoming virtual extensions of [human] memory." The remainder of the passage details Wegner's experiments and findings. Thus, the main purpose of the passage is to share the findings of a study examining the effect of computer use on memory recall.
Choice A is incorrect. Although the author suggests in the sixth paragraph that technology may interfere with critical thinking, this isn't the focus of Wegner's experiments, nor is illustrating this position the main purpose of the passage. Choices B and C are incorrect because the passage doesn't support the assertion that people have become overly dependent on computers for storing information (choice B) or that humans' capacity for memory is much weaker that it once was (choice C).