The correct option is
C mRNA
m-RNA is known as messenger RNA. It has act as a template for synthesis of protein by the process of translation. The m- RNA obtained by the transcription of DNA has the coding and noncoding region. The noncoding region is primed of during post translational modification. The sequence of base on the m-RNA template determines the sequence of protein.
The m-RNA has complete nucleotide sequence so it is considered as largest RNA while t-RNA is the smallest one.
So, the correct answer is option A.