Answer: The correct answer is "to take into mind or memory"
Explanation:
Following the definition provided by The Cambridge Dictionary, DIGEST means "to take information into your mind in a way that gives you the ability to use it." The example provided in the dictionary is the following:
"He could digest an enormous amount of information with amazing speed."
So he can memorize pieces of information very easily.
In the example "Those names and dates and places-such a challenge to digest!" It seems the person is having a hard time memorizing information. If you continue reading, the person says that he tends to forget things when he's taking a quiz. Then, he adds his head is "full of cotton", which may mean he does't have have anything (cotton) in his head after he studied.
He finds it very hard to digest or memorize information, in this case, names , dates and places of the history lesson.
