Final answer:
The chemical difference between eukaryotic and prokaryotic mRNA is the presence of a 5' (7-methyl guanosine) cap in eukaryotic mRNA, which aids in protection, export from the nucleus, and translation initiation.
Step-by-step explanation:
The chemical difference between a eukaryotic mRNA and a prokaryotic mRNA is C. the 5' (7-methyl guanosine) cap. This cap is a modification added to the 5' end of eukaryotic pre-mRNA molecules and is not found in prokaryotic mRNAs. This 5' cap consists of a 7-methyl guanosine nucleotide that is linked to the pre-mRNA by an unusual 5',5'- triphosphate linkage. The presence of this cap helps in the protection of the eukaryotic mRNA from degradation and is also involved in its export from the nucleus and in the initiation of translation.
Option D, mentioning the use of arsenic, is incorrect as arsenic does not replace phosphorus in RNA. Option B is also incorrect because both eukaryotic and prokaryotic mRNA contain uracil instead of thymine, which is found in DNA. Lastly, option A is a mischaracterization; no adenine residues are added at the 5' end but at the 3' end forming what is known as the poly-A tail.