The correct option is C Summary
A synonym is a word that is similar in meaning to another.
The given word 'synopsis' means 'a brief summary or an outline of a play, story or chapter'. For example, 'The synopsis of the book is written on its back cover.'
From the options, 'summary' means the same thing: a brief account of the main points of something. Consider the sentence- The summary of the chapter is given on the front page. Hence 'summary' is a synonym of 'synopsis' and option D is correct.
'Index' is an alphabetical list of names or chapters with reference to their page numbers in a book. It refers to names and titles, and not the content of those titles. Hence it isn't a synonym of 'synopsis' and option A is incorrect.
'Mixture' is a substance that is made by blending other substances together. It does not mean the same as 'synopsis', hence option B is incorrect.
4. 'Puzzle' refers to something that is difficult to understand. It does not mean the same as 'synopsis', hence option C is incorrect.