The correct answer is: B. peculiar.
Peculiar means “strange or unusual in a way that is interesting or amusing.” It is the most accurate description of the way that some millionaires become eccentric in their old age.
Irritable means “easily annoyed or angered.” This is not always the case with eccentric millionaires. Some of them may be irritable, but others may be quite pleasant.
Dull means “not interesting or exciting.” This is also not always the case with eccentric millionaires. Some of them may be dull, but others may be quite interesting and engaging.
Miserly means “unwilling to spend money or give to others.” This is not always the case with eccentric millionaires. Some of them may be miserly, but others may be quite generous.
In conclusion, the correct answer is B. peculiar.