The correct option is C rRNA
DNA polymerase enzyme adds deribonucleotides to synthesize DNA using the base sequence of parental DNA strand in nucleus. Option A is incorrect. Primary transcript is synthesized by RNA polymerase during transcription in nucleus which undergoes processing to produce a functional mRNA. Option B is incorrect. Transfer RNAs are synthesized by transcription of tRNA gene in nucleus by by RNA polymerase; option D is incorrect. The darkly stained body in the nucleus is nucleolus which is involved in rRNA synthesis and ribosome construction. Correct option is C.