Find the synonym of the underlined word. 

What a red-letter day it was! Five victories in five events! 

A red-letter day is a special occasion or chance. Its origins can be traced back to classical antiquity; for example, important days are highlighted in red on a Roman Republic calendar. Initial caps and highlighted words were written in red ink in medieval manuscripts.
We can deduce from the preceding explanation that the letter' indicates memorable. Whereas Neon implies the noble gas group's inert gaseous element with atomic number 10, carbon. It is made by distilling liquid air, and it is utilised in fluorescent lighting, neon, and other gas signs, as well as illuminated advertising signs.
"The hotel's foyer was lit up with neon."

As a result, option D is valid and the other options are incorrect because they do not give the necessary information.