(b) Transactional websites for search terms that denote possible purchase intent, or other websites that are action-oriented (e.g., Yelp reviews).
(c) More “user-friendly” explanation sites (e.g., for medical terminology, a website that explains it in a more friendly style, or WikiHow)
(d) Subject-specific references (some overlap with (c), but could also include domain Wikias, or other wikis)
(e) When the search term is trending because of a recent news item, then links to the news item (even if the search query itself does not specify the associated news)
Varies heavily by context. Typical alternatives:
(a) Google’s own answers for simple questions.
(b) Transactional websites for search terms that denote possible purchase intent, or other websites that are action-oriented (e.g., Yelp reviews).
(c) More “user-friendly” explanation sites (e.g., for medical terminology, a website that explains it in a more friendly style, or WikiHow)
(d) Subject-specific references (some overlap with (c), but could also include domain Wikias, or other wikis)
(e) When the search term is trending because of a recent news item, then links to the news item (even if the search query itself does not specify the associated news)