The correct option is
A Summary
'Compendium' means a short but complete account of a particular subject, especially in the form of a book.
Option B - 'Index' means an alphabetical list, such as one printed at the back of a book showing which page a subject, name etc is on. Thus option B is incorrect.
Option C - 'Reference' means a mention of something. Thus option C is incorrect.
Option D - 'Company' is an organisation that sells goods or services in order to make money. Thus option D is incorrect.
Option A - 'Summary' means a short, clear description that gives the main facts or ideas about something. Thus option A is the correct answer.
'Compendium - Summary'