It is very difficult to retain all that you hear in the class.

Answer: Option A

keep : have or retain possession of.
recall : bring (a fact, event, or situation) back into one's mind; remember.
preserve : maintain (something) in its original or existing state.
conserve : protect (something, especially something of environmental or cultural importance) from harm or destruction.
retain : continue to have (something); keep possession of.


Synonym of retain is keep