It is impossible for an ordinary mortal to . . . . . . . . all these figures to memory.

The correct answer is C. retain.

To retain something means to keep it in one’s mind or memory. In this case, it is impossible for an ordinary mortal to keep all these figures in their memory.

The other options are not as accurate.

• Keep can mean to hold or possess something, but it does not necessarily mean to remember it.
• Commit can mean to promise to do something, but it can also mean to memorize something. In this case, it is unlikely that an ordinary mortal would be able to commit all these figures to memory.
• Hoard can mean to collect or store something, but it does not necessarily mean to remember it.