The Roman numeral for 51 is LI, representing 50 (L) + 1 (I). Option A (IL) would represent 50-1=49 if valid, but subtractive notation only allows certain combinations. Option B (LX) is 60, and Option D (XL) is 40. Roman numerals combine symbols additively, with specific subtractive rules for 4, 9, 40, 90, etc.