In the process of transcription RNA polymerase is only capable of elongation. Initiation factor sigma helps in the process of initiation while termination factor Rho is helping in termination of the process.
So, the correct answer is 'Elongation'.

RNA polymerase is an enzyme that is responsible for catalyzing the process of transcription, which is the process of synthesizing RNA from a DNA template. The enzyme is capable of catalyzing three distinct stages of transcription, including initiation, elongation, and termination. However, the question asks which of these stages the enzyme is only capable of catalyzing, which is the stage of elongation.

Elongation
Elongation is the stage of transcription in which the RNA polymerase enzyme adds nucleotides to the growing RNA chain, extending it in a 5' to 3' direction. During this stage, the enzyme moves along the DNA template strand, unwinding it ahead of the active site and rewinding it behind the active site. The enzyme catalyzes the formation of phosphodiester bonds between adjacent nucleotides, creating a new RNA molecule that is complementary to the DNA template.

Initiation
Initiation is the stage of transcription in which the RNA polymerase enzyme binds to a specific region of DNA known as the promoter. The enzyme then unwinds the DNA to expose the template strand, allowing it to begin synthesizing RNA. During initiation, the enzyme also recruits other proteins and factors that help to regulate and coordinate the transcription process.

Termination
Termination is the final stage of transcription in which the RNA polymerase enzyme reaches the end of the DNA template and releases the newly synthesized RNA molecule. Termination can occur in several different ways, depending on the specific type of RNA being synthesized and the regulatory signals present in the DNA.

Conclusion
In summary, while RNA polymerase is capable of catalyzing all three stages of transcription (initiation, elongation, and termination), the enzyme is only capable of catalyzing the process of elongation on its own, without the assistance of other proteins or factors. Initiation and termination require additional regulatory signals and proteins to properly coordinate the transcription process.