Answer:
The line which clearly shows that the theme of the story is that (C) "Marta gasped when she discovered the necklace was no longer around her neck".
Step-by-step explanation:
This line illustrates the continuance of the plot and the actions which Marta took when she realised that her necklace is lost. She then goes into debt for the artificial necklace for ten years and ruins her life over nothing. The "theme" of "appearance vs reality" is thus illustrated with the help of the line mentioned above.