A transcription unit in DNA is not defined primarily by:

The correct answer is Option (2) → Inducer

A transcription unit is the stretch of DNA that is copied into one RNA. It has three regions: a promoter, a structural gene and a terminator.

(1) Terminator — Defines a transcription unit. It lies at the 3′ end of the coding strand and marks where transcription stops.

(2) Inducer — Does not define a transcription unit. An inducer is a small molecule, such as lactose in the lac operon, that switches a gene on. It is not a region of the DNA.

(3) Promoter — Defines a transcription unit. It is the site where RNA polymerase binds, and it lies at the 5′ end of the structural gene.

(4) Structural gene — Defines a transcription unit. It is the part of the DNA that is actually transcribed.

NCERT Extract:
Transcription Unit – “A transcription unit in DNA is defined primarily by the three regions in the DNA : (i) A Promoter (ii) The Structural gene (iii) A Terminator There is a convention in defining the two strands of the DNA in the structural gene of a transcription unit.”
The Lac Operon – “… Lactose is the substrate for the enzyme beta-galactosidase and it regulates switching on and off of the operon. Hence, it is termed as inducer.”