Which of the following mechanisms of gene regulation is epigenetic?

Which of the following mechanisms of gene regulation is epigenetic?

a. A transcriptional regulatory protein is produced only when phenylalanine levels are low, and binds to enhancer regions of genes involved in phenylalanine biosynthesis to activate their transcription.

b. Tissue-specific patterns of nucleosome acetylation are established near a gene promoter early in development, and are perpetuated through subsequent rounds of cell division.

c. Different transcription factors bind to heart-specific and lung-specific enhancer regions to activate gene transcription in these tissues.

d. A hormone binds to a transcription factor, making it able to bind DNA and activate transcription when  sucrose is present.