In E. coli, a cluster of three genes called the lac operon
produce enzymes that break
apart lactose. To the side of the genes is a region called the Choose...
, where RNA
polymerase binds, and another region called the Choose...
where the lac repressor
binds.

the correct answer is option d.

an appositive is a noun, noun phrase or noun clause which follows another noun or noun phrase in order to describe it or name it another way. they are usually used between commas, brackets or dashes.

the example sentence with the correct use of commas is the following:

her uncle, the man over there in the red shirt, owns the hardware store in town.