The best summary of the scene in Chapter 5 is:
C. The dock's green light, which represents Gatsby's idealized love for Daisy, has lost its enchantment now that she is beside him.
Explanation:
"Possibly it had occurred to him that the colossal significance of that light had now vanished forever."
That is what Nick, the narrator, says once the scene unravels. The main character, Gatsby, spent five years dreaming of gaining back the love of his life, Daisy. As he got closer to his dream's completion and moved to a mansion across the bay from hers, Gatsby focused on a green light that shone from Daisy's dock.
The light acquired a meaning, representing Gatsby's goal. He idealized Daisy for all those years, working hard - even becoming a criminal - just to get her back. And now that he finally has her, there is nothing to hope for anymore. No more dream, no ideals, nothing. The light has just lost its significance.