Just by looking at the question and answers, you can determine that the most formal statement is A. A man of disreputable nature. By the use of the word formal, people tend to think of respectful/professional words/things. The words shady character and snake in the grass are more of a casual/informal way of describing a "suspicious" person.
Most of the time, you'll want affect as a verb meaning to influence something and effect for the something that was influenced. The difference between affect and effect is so slippery that people have started using "impact" as a verb instead. Don't be one of them!
