Explanation:

The correct answer is option 'D' - promoter.

Operon:
An operon is a cluster of genes that are transcribed together as a single mRNA molecule. This mRNA molecule is then translated into multiple proteins. The operon consists of three main components: the promoter, the operator, and the structural genes.

Promoter:
The promoter is a region of DNA located upstream of the structural genes. It is responsible for initiating the transcription process. RNA polymerase, the enzyme responsible for synthesizing RNA, binds to the promoter region to begin transcription.

Role of RNA Polymerase:
RNA polymerase is the enzyme that carries out the transcription process. It binds to the DNA template strand and synthesizes a complementary RNA molecule. It recognizes and binds to the promoter region to initiate transcription.

Binding of RNA Polymerase:
During the expression of an operon, RNA polymerase specifically binds to the promoter region of the operon. This binding is facilitated by the recognition of specific DNA sequences within the promoter region by the RNA polymerase.

Importance of Promoter Binding:
The binding of RNA polymerase to the promoter region is crucial for the initiation of transcription. Once bound, RNA polymerase can start synthesizing an mRNA molecule using the DNA template strand. This mRNA molecule will contain the genetic information encoded by the structural genes within the operon.

Regulator Gene and Operator:
The regulator gene is a separate gene that is not a part of the operon. It encodes a regulatory protein called a repressor, which can bind to the operator region. The operator is a region of DNA located within the operon, between the promoter and the structural genes. The repressor protein can block the binding of RNA polymerase to the promoter region by binding to the operator.

Therefore, during the expression of an operon, RNA polymerase binds to the promoter region to initiate transcription. The regulator gene and operator play a role in regulating the expression of the operon by controlling the binding of RNA polymerase to the promoter.