The correct option is B seventh
The number of members in each generation in a family tree is double the number of members in the previous generation.
So, starting from current generation with 1 member,
First generation =1×2=2 members
Second generation =2×2=4 members
Third generation =4×2=8 members
Fourth generation =8×2=16 members
Fifth generation =16×2=32 members
Sixth generation =32×2=64 members
Seventh generation =64×2=128 members
Therefore, the seventh generation of a family tree has 128 members.