Answer
See below
Work Step by Step
We can notice that Cheryl is in generation 1. Thus $a_1=1$
Her parents are in generation 2, $a_2=2$
She has 4 grandparents, $a_3=4$
Hence, the sequence can be:
$$a_1=1=2^0=2^{-1}\\a_2=2=2^1=2^{2-1}\\a_3=4=2^2=2^{3-1}$$
The matching rule is $a_n=2^{n-1}$
The correct answer is B.
