Final answer:
The mechanism illustrating transcriptional repression unique to eukaryotes is repressors recruiting enzymes to modify histone groups to compact chromatin, therefore option (a) is the correct answer. This process is part of the epigenetic regulation and decreases gene expression by limiting access of transcriptional machinery to DNA.
Step-by-step explanation:
Transcriptional repression in eukaryotes can occur through several mechanisms, one of which is unique to them due to the complexity of their chromatin structure. The correct option that illustrates a form of transcriptional repression unique to eukaryotes is (a) Repressors recruiting enzymes to modify groups on histones to compact chromatin. This mechanism involves the modification of histone proteins, around which DNA is wound, altering the accessibility of genes to the transcriptional machinery.
Unlike prokaryotes, eukaryotic cells utilize histone modification as a way to regulate gene expression. These modifications can change the conformation of chromatin from an open (euchromatin) to a closed (heterochromatin) state, making it inaccessible to transcription factors and RNA polymerase. Thus, when repressors recruit enzymes to add chemical groups such as methyl or acetyl groups to histones, they either loosen or tighten the chromatin structure. The option unique to eukaryotes is the tightening, or compaction, which inhibits gene expression.