The cI repressor when active (a) Represses the lysogenic phase (b) Represses the lytic phase (c) Plaques are observed (d) Represses the viral polymerase

The cI repressor, when active, (b) represses the lytic phase.

Here's a step-by-step explanation:

1. The cI repressor is a protein that plays a crucial role in the life cycle of bacteriophage lambda, a virus that infects bacteria.

2. This protein is produced when the bacteriophage enters the lysogenic phase, a stage where the virus integrates its DNA into the host's genome and lies dormant.

3. When the cI repressor is active, it binds to specific sites on the phage's DNA, preventing the transcription of genes that would lead the phage into the lytic phase.

4. The lytic phase is a stage where the virus hijacks the host's machinery to replicate its own DNA and produce new virus particles. This phase ends with the lysis (or destruction) of the host cell, releasing the newly formed viruses.

5. Therefore, by preventing the phage from entering the lytic phase, the cI repressor effectively represses this phase.

6. This does not mean that plaques are observed (c), as plaques are clear areas that appear on a lawn of bacteria following lysis by phages. Since the cI repressor prevents the lytic cycle, it also prevents plaque formation.

7. The cI repressor also does not repress the viral polymerase (d). The viral polymerase is an enzyme that the virus uses to replicate its DNA during the lytic phase. While the cI repressor does prevent the virus from entering the lytic phase, it does this by blocking the transcription of the genes necessary for this phase, not by directly repressing the viral polymerase.