Imagine that you've discovered a eukaryotic regulatory sequence thousands of base pairs away from the promoter, and that when this sequence is lost due to mutation, levels of gene expression decrease sharply. This sequence is . Select one: a. a promoter b. a promoter-proximal element c. a silencer d. an enhancer

The correct answer is option d, that is, an enhancer.

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.