Thymine would never be part of an mRNA strand.thus, option A is correct.
What is mRNA?
Messenger RNA (mRNA), molecule in cells that carries codes from the DNA in the nucleus to the sites of protein synthesis in the cytoplasm (the ribosomes).
It was also known that there are only four nucleotides in mRNA: adenine (A), uracil (U), guanine (G), and cytosine (C). Same three nucleotide base is also present in the DNA except thymine. Thymine is replaced with uracil in case of RNA. thus option A is correct.
Learn more about RNA here:
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