Final answer:
In eukaryotic cells, gene expression regulation occurs during transcription, which takes place in the nucleus and translation occurs in the cytoplasm.
Step-by-step explanation:
In eukaryotic cells, gene expression regulation occurs during transcription, which takes place in the nucleus. The DNA in the nucleus is transcribed into mRNA, which is then modified and transported out of the nucleus into the cytoplasm. In the cytoplasm, ribosomes translate the mRNA into protein. The processes of transcription and translation are physically separated by the nuclear membrane, with transcription occurring only within the nucleus and translation occurring only in the cytoplasm. In a eukaryotic cell, regulation of gene expression during transcription takes place in the nucleus. This is because eukaryotic DNA is housed within the cell's nucleus, where it is transcribed into mRNA. This mRNA is then eventually transported out of the nucleus and into the cytoplasm, where translation into proteins occurs at the ribosomes. As such, for the question provided, the correct answer would be: A) Nucleus.