a1 = 9/20
a6 = 8/135
an = a1 . q^(n - 1)
a6 = a1 . q^5
8/135 = 9/20 . q^5
q^5 = 8/135 . 20/9
q^5 = 160/1215
q = 2/3
Now let's multiply a1 by it a couple times.
a1 = 9/20
a2 = 9/20 . 2/3 = 18/60 = 3/10
a3 = 3/10 . 2/3 = 6/30 = 3/15
a4 = 3/15 . 2/3 = 6/45 = 2/15
a5 = 2/15 . 2/3 = 4/45
a6 = 4/45 . 2/3 = 8/135
The only ones that appear on the alternatives are 4/45, 2/15 and 3/10.