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 <em>digest or memorize information, in this case, names , dates and places of the history lesson.</em>
