Answer:
The correct answer is option d, that is, an enhancer.
Step-by-step explanation:
An enhancer refers to a short sequence of DNA, that is, about 50 to 1500 base pairs in length, which performs an essential role in enhancing the transcription process of a specific gene. These enhancers are also termed as transcription factors.
An enhancer is found within the gene's coding region and can be present downstream of a gene, upstream of a gene, or thousands of base pairs away from a promoter sequence. As its primary role is to increase the process of transcription, thus, if anyhow, an enhancer is lost because of a mutation, the expression of a gene may even decline at a brisk rate.