Messenger RNA is a type of RNA which carries information from the synthesis of protein transcribed from a DNA template. In DNA, there are four different bases: adenine (A), guanine (G), cytosine (C) and thymine (T). Each base has its complementary base, which means in the double helical structure of DNA, A will have T as its complimentary and similarly G will have C. mRNA is formed as a complementary strand to one of the two strands of the DNA. Three of the four nitrogenous bases that make up RNA — adenine (A), cytosine (C), and guanine (G) — are also found in DNA. In RNA, however, a base called uracil (U) replaces thymine (T) as the complementary nucleotide to adenine. This means that during elongation, the presence of adenine in the DNA template strand tells RNA polymerase to attach a uracil in the corresponding area of the growing RNA strand. The mRNA formed will be complimentary to the DNA strand. During transcription RNA polymerase begins moving down the DNA template strand in the 5' to 3' direction, when it does it strings together complementary nucleotides. Because of complementary base- pairing, this action creates a new strand of mRNA that is organized in the 3' to 5' direction. As the RNA polymerase continues down the strand of DNA, more nucleotides are added to the mRNA, thereby forming a progressively longer chain of nucleotides. This process is called elongation. So, the correct answer is option C.