Ascientist is studying the action of rna polymerase during transcription. he wants to know what type of bases are added to the growing mrna by rna polymerase.

A process of copying DNA sequence into messenger RNA (mRNA) is called transcription. RNA polymerase is an enzyme involved in this process. First, it binds to DNA and separates its two strands. Then, it adds ribonucleotides to the complementary nucleotides on DNA strand and, thus, it makes mRNA from DNA. Afte RNA polymerase de-attaches from DNA strand, transcription of mRNA stops and mRNA is now free. 

I’m going to go with b!