RNA polymerase catalyzes the addition of many rNTPs to a growing RNA.

a. initiation

b. elongation

c. termination

b. Elongation

Explanation:

During the start of the elongation phase of RNA synthesis, the sigma subunit of RNA polymerase of E. coli is released and NusA protein takes its place. This makes the RNA polymerase to move away from the promoter.

During the elongation phase of RNA synthesis, the enzyme DNA dependent RNA polymerase elongates the RNA strand by addition of ribonucleotides to the free 3'-OH end. The process occurs in 5' to 3' direction and the 3'-OH group serves as a nucleophile.