In RNA, adenine will pair with
The structure of thymine and uracil are almost identical, except for the methyl group replacing a hydrogen on the ring. This methylation carries with it two points: 1) the molecule is more stable, and 2) more energy was needed to form thymine. Since RNA is generally shorter-lived and more mass-produced than DNA, it would be more energetically efficient to use uracil rather than thymine as a base-pair with adenine.
Adenine would readily pair with either base, but in RNA it is more energy-efficient to utilize uracil over thymine (whereas in DNA thymine is more useful as the methylation protects against nucleases and erroneous replacement of thymine with cytosine).
Therefore, the correct answer is option C.