The correct option is B MMMCMXCIX
3999 = 3000 + 900 + 90 + 9
3999 = 1000 + 1000 + 1000 + (1000 - 100) + (100 -10) + (10 -1)
This can be written in numeral form as:
(M + M + M ) + ( M - C ) + ( C - X) + (X - I)
= MMMCMXCIX
Therefore, option b is correct.