The correct answer would likely be:
c. genes 1 and 2 regulate the expression of gene 3.
A master control gene often acts as a regulatory element, affecting the expression of other genes in the pathway. If a change occurs in the master control gene, it could disrupt the regulation of genes that are downstream in the pathway, such as genes 1 and 2, which in turn could affect the expression of gene 3.
Without specific details about the diagram and the relationships in question, this answer is based on common genetic regulatory mechanisms, where changes in a master control gene can impact multiple downstream genes.