It would be UGCCAG. This is because in mRNA, thymine is replaced by Uracil. That means the complementary base pairings are uracil and adenine (instead of thymine and adenine in DNA) and cytosine and guanine. So you need to change them to the other base in the pair. Meaning A (adenine) becomes the base it is paired with, being Uracil in this case and so on.
