Answer:
If a son were to have a grandchild, Bob and Fran would then have two great-grandchildren.
Step-by-step explanation:
C = children
G = grand children
GG = great grand children
Bob and Fran have 3 children, 2 sons(C) and 1 daughter(C).
Their 2 sons have 5 children(G) between them. One son has 2 children(G). Their daughter has 3 children(G) and 1 grandchild(GG).
If the son with 2 children had a grandchild(GG).
Now we can see that, at present we have a single great grand child that is of their daughter's.
As given, if a son were to have a grandchild, Bob and Fran would then have two great-grandchildren.