Answer:

b

Step-by-step explanation:

an = a1 (r)^n-1

a3 = 4 (r)^3-1

100 = 4 (r)^2

----- = ---------

4           4

25 = r^2

5 = r

an = a1 (r)^n-1

a2 = 4 (5)^2-1

a2 = 4 (5)^1

a2 = 20