Final answer:
The number 999 in French is written as Neuf cent quatre-vingt-dix-neuf.
Step-by-step explanation:
The number 999 in French is written as Neuf cent quatre-vingt-dix-neuf. It is composed of three parts: neuf cent (900) and quatre-vingt-dix-neuf (99). In French, the numbers from 70 to 99 are formed using a combination of the words for multiples of ten (soixante, quatre-vingt) and the numbers one to nine (dix, onze, douze, treize, etc.). Therefore, the correct answer is option B: Neuf cent quatre-vingt-dix-neuf.
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