The correct option is
B Summary
The given word 'synopsis' means 'a brief summary of the plot of a novel'.From the options, Option C: 'summary' best expresses the meaning of the given word. Hence option C is correct.
Option A: 'Index' in a book refers 'to an alphabetical list of names or topics discussed in that book'.
Option B: 'Mixture' means 'a combination or blend of different elements'.
Option D: 'Puzzle' means 'a toy or a problem designed to amuse the person and something which has a logical solution to it'.
These words do not mean the same as the given word, hence options A, B and D are incorrect.