Final answer:
Shawn and Heather are first cousins and share 1/8 (option C) of their genes in common, as their mothers are sisters, each contributing genetically to their children.
Step-by-step explanation:
Shawn's mother and Heather's mother are sisters. This means that Shawn and Heather are first cousins. In terms of genetics, first cousins share 1/8 of their genes in common. Each parent contributes 50% of their genes to their children. Since Shawn's and Heather's mothers are sisters, they each received half of their genetic makeup from their own parents. When the cousins' shared grandma and grandpa are considered, it means that Shawn and Heather each have received 1/4 of their genes from the same two people (their shared grandparents), and since genes are diversified with each generation, this further breaks down to them sharing approximately 1/8 of the same genes.