A certain regulatory gene is involved in controlling the expression of other genes. In studying this regulatory gene, a scientist discovers that it encodes for an enzyme that causes histone deacetylase (the removal of acetyl groups from histone tails). Based on this information, how would you predict that this enzyme regulates gene expression

by an epigenetic mechanism

Explanation:  

Epigenetics can be defined as the study of mitotically heritable gene expression changes that do not involve alterations in the DNA sequence. There are three different epigenetic modifications that work together either to activate or repress gene expression: DNA methylation, histone modifications (e.g., methylation, acetylation, phosphorylation, etc) and regulatory non-coding RNAs (e.g., long non-coding RNAs, microRNAs, piwi-RNAs, etc). Deacetylases are enzymes that play roles in epigenetic gene regulation by removing acetyl groups from lysine (K) residues located at the N-terminal end of histones H3 and H4. Histone deacetylation modifies the structure of the chromatin, thereby activating or suppressing gene expression in a context-dependent manner.

the answer would be..

b. sucking