The correct answer is uracil.
Uracil is a nitrogen base that is found in the RNA and not in the DNA, which has thymine. The three other nitrogen bases are the same ( Adenine, Guanine and Cytosine).
Besides the difference in the nitrogen bases, the two molecules also differ in the sugar that builds the structure ( DNA has desoxyribose and RNA has ribose) and the DNA is a two stranded helix, whereas the RNA is a single stranded molecule.