During expression of lac operon, RNA polymerase binds to which one of…

The correct answer is Option (3) → Promoter region.

The lac operon has a promoter (p), an operator (o) and three structural genes (z, y and a), with a separate regulator gene (i). The enzyme that copies genes into RNA, RNA polymerase, binds at the promoter. When lactose (the inducer) is absent, the repressor sits on the operator and blocks RNA polymerase. When the inducer is present, the repressor is inactivated and RNA polymerase can reach the promoter and start transcription.

Option (1) Operator region. — Incorrect. The operator is where the repressor binds. When the repressor sits there, it stops RNA polymerase from transcribing the operon.

Option (2) Inducer. — Incorrect. The inducer (lactose or allolactose) is a molecule, not a DNA region. It binds the repressor and inactivates it.

Option (3) Promoter region. — Correct. The promoter is the DNA site where RNA polymerase binds and starts transcription. In the presence of the inducer it gets access to the promoter.

Option (4) Structural genes. — Incorrect. The structural genes (z, y and a) are the parts that are copied into lac mRNA. RNA polymerase moves along them after it has bound at the promoter.

NCERT Extract:
The Lac Operon – “… The repressor protein binds to the operator region of the operon and prevents RNA polymerase from transcribing the operon. In the presence of an inducer, such as lactose or allolactose, the repressor is inactivated by interaction with the inducer. This allows RNA polymerase access to the promoter and transcription proceeds (Figure 5.14).”
Types of RNA and the Process of Transcription – “… RNA polymerase binds to promoter and initiates transcription (Initiation).”

Figure 5.14 The lac Operon