Questions: Adenine Cytosine Guanine Uracil Thymidine

The answer is the fourth one: Uracil.

Explanation for each option:

1. Adenine - This is a nucleotide base found in both DNA and RNA, so it is not specific to RNA.
2. Cytosine - This is also a nucleotide base found in both DNA and RNA, so it is not specific to RNA.
3. Guanine - Like adenine and cytosine, guanine is found in both DNA and RNA.
4. Uracil - This is a nucleotide base that is specific to RNA. In DNA, thymine is used instead of uracil.
5. Thymidine - This is a nucleotide base found in DNA, not RNA. In RNA, uracil is used instead of thymine.

Summary: Uracil is the nucleotide base that is specific to RNA, distinguishing it from DNA, which uses thymine instead.