Final answer:
In the given quote, the #3 reference mark is referring to the word 'citadel,' which metaphorically suggests that Dunbar was an important center of education during segregation.
Step-by-step explanation:
The word that the #3 reference mark is referring to in the quote "Dunbar was a citadel of learning in segregated Washington" is B) citadel. The reference mark is commonly used to draw attention to a specific word or phrase, and in this context, it appears that "citadel" is the term being highlighted. A citadel is a fortress, often on high ground, protecting or dominating a city. In this quote, it metaphorically suggests that Dunbar was a stronghold or important center of education during a time when society was segregated.