The correct option regarding the lac operon in E.coli from the following is

The correct option is "lac operon messenger RNA is a polycistronic mRNA".

Step-by-step Explanation:

1. Understanding Polycistronic mRNA:
In prokaryotes like Escherichia coli (E. coli), structural genes that are functionally related are often grouped together and regulated under a single promoter. When these genes are transcribed, they produce a single, continuous messenger RNA (mRNA) molecule that contains the coding sequences for multiple distinct proteins. Such an mRNA is referred to as polycistronic mRNA. In contrast, eukaryotes typically produce monocistronic mRNA, where one mRNA molecule encodes only a single polypeptide.

2. Structure of the Lac Operon:
The lac operon consists of a promoter (P), an operator (O), and three structural genes:
- lacZ: Encodes the enzyme β-galactosidase.
- lacY: Encodes lactose permease.
- lacA: Encodes transacetylase.
During transcription, all three structural genes (lacZ, lacY, and lacA) are transcribed together into a single, polycistronic mRNA. This allows the cell to coordinate the expression of all enzymes needed for lactose metabolism simultaneously.

3. Analyzing why the other options are incorrect:
- "Lac operon is switched on in the absence of lactose" is incorrect because in the absence of lactose (the inducer), the repressor protein binds to the operator region, preventing transcription and keeping the operon switched off.
- "Lac repressor binds to the lac promoter" is incorrect because the lac repressor specifically binds to the operator region, not the promoter region. The promoter is the binding site for RNA polymerase.
- "β-galactosidase is the only enzyme produced in large quantities when lac operon is turned on" is incorrect because the operon produces all three enzymes (β-galactosidase, permease, and transacetylase) when it is switched on, as they are all transcribed from the same polycistronic mRNA.