In English grammar, an abstract noun describes something you cannot see, hear, touch, taste or smell. Examples of abstract nouns are feelings or emotions, states or attributes, concepts or events. Example: hatred, beauty, brilliance, culture, knowledge, leisure, hospitality etc.
The word 'inform' means to provide information to someone.
The abstract noun for inform is information which means knowing something or having learned some facts. Therefore, the correct answer is option D) information.
Option B) informative is an adjective which means providing some fact or interesting information.
Option C) informer is a person who provides information.