The nitrogenous bases are Adenine (A), Guanine (G), Thymine (T), Cytosine (C), and Uracil (U). A pairs with T in DNA and with U in RNA. C always pairs with G. This question is tricky because not only does it want you to find the complementary strand, but it wants you to find the complementary MRNA strand when it gives you a DNA strand. So let's break it up.
When you go from DNA to RNA, all of the Thymines turn into Uracils.
When you look at a strand of DNA or RNA, you can figure out what the complementary strand is by writing the match to each base on the parent strand (which is the strand you were given). So, if your strand of DNA gave you a C, you know it will be matched to a G on the complementary strand.
Let's start with the complementary strand for the DNA strand you were given first. We know A matches with T, and G matches with C. So your DNA strand would be TTT/AAA/CCC/GGG. But we aren't done yet, because we need to find what this strand is in MRNA. To do this, we need to change all of the Thymines to Uracil. So, your strand would actually be UUU/AAA/CCC/GGG.
I hope this helps!