The enzyme that adds new nucleotides to build mRNA is called

RNA polymerase

Explanation:

The polymerase then aligns the correct nucleic acid (A, C, G, or U) with its complementary base on the coding strand of DNA. RNA polymerase is an enzyme that adds new nucleotides to a growing strand of RNA. This process builds a strand of mRNA.

answer; /// i believe the correct answer is (reactants);