详解
Choice B is the best answer. In the third paragraph, Wegner describes transactive memory as a "network of memory where you don't have to remember everything in the world yourself." Instead, the burden of storing information is shifted to transactive memory sources that can function as "extensions of [human] memory." Examples of sources provided in the fourth paragraph of the passage include cell phones, GPS devices, and search engines. What these examples have in common is that they store information, such as phone numbers, directions, and general knowledge, so that a person doesn't have to commit this information to memory. A written list of a user's passwords for different websites serves the same function as these examples. Although remembering a list of passwords for different websites is conceivable without a list, keeping such a list shifts the burden of storing readily memorable information away from the user because the list preserves the information in place of the user's memory. Therefore, based on the passage, a written list of a user's passwords for different websites would be considered a transactive memory source.
Choices A, C, and D are incorrect because they don't accurately exemplify transactive memory sources. A souvenir brought home from a memorable trip (choice A) may evoke memories of that place for the owner. However, it doesn't preserve actual information in the way the examples provided in the passage do. A library database that helps users locate specific books (choice C) may seem similar to a search engine. However, it doesn't store information that would otherwise be readily memorable in the way that a search engine can help a user remember an actor's name or a detail of geography, according to the fourth paragraph of the passage. Instead, it helps a library patron navigate a system that is typically far too vast to be committed to memory. A website that helps users plan and make travel arrangements (choice D) may help facilitate transactions in the form of ticket purchases or hotel reservations. However, it doesn't store information that the user would otherwise memorize.