RNA differs from DNA in having

DNA (Deoxyribonucleic acid) uses four "letters" or bases to write its instructions: adenine (A), guanine (G), cytosine (C), and thymine (T).
RNA (Ribonucleic acid) is very similar, but it has a slight change in one of its letters.

Let's look at the options:
* Option A: thymine: DNA does have thymine (T). RNA doesn't use thymine.
* Option B: uracil: RNA uses uracil (U) instead of thymine. DNA doesn't have uracil.
* Option C: adenine: Both DNA and RNA have adenine (A).
* Option D: guanine: Both DNA and RNA have guanine (G).

The key difference is that DNA uses thymine (T), while RNA uses uracil (U).

Therefore, the correct answer is B: uracil.