Answer:
D. the nature of people to blame others for their own mistakes or shortcomings
Step-by-step explanation:
The idea behind this excerpt can be noticed right here: "when they find they've given a tramp a million-pound bill when they thought it was a one-pounder, they are in a frantic rage against him instead of quarreling with their own near-sightedness".
The character is clearly describing that once they find out what they have done, they're going to search someone to blame. Most likely the one who took the a million-pounder for taking the note, even though it was given to him, not stolen.