The best statement that explains how protein X regulates gene expression is:
b. Protein X is responsible for activating transcription of some genes but not others.
This statement suggests that protein X plays a role in the regulation of gene expression by selectively activating the transcription of certain genes. Transcription is the process by which the information encoded in a gene is transcribed into messenger RNA (mRNA) molecules. By activating transcription, protein X can control which genes are expressed and which ones are not, thus regulating gene expression. This selective activation of transcription allows for precise control and regulation of gene expression patterns in response to specific cellular signals or environmental cues.