Here are the odd words in each line:
b) informal - The other words (information, incorrect, irregular) are adjectives related to correctness or classification, while "informal" specifically refers to a casual style or manner.
c) guilty - The other words (activity, possibility, similarity) are all nouns, while "guilty" is an adjective that describes a state or condition.
d) successful - The other words (happiness, membership, brotherhood) are all nouns, whereas "successful" is an adjective that describes a condition or state of achievement.