In mathematics, a ratio indicates how many times one number contains another.
in this case, the ratio is 4
1)
2, 6, 10, 14, ...
2*4 = 8, which is different to 6 so the ratio is different to 4
2)
1, 4, 16, 64,...
1*4 = 4
4*4 = 16
16*4 = 64
In this case, the sequence has a common ratio of 4
3)
1600, 400, 100, 25, ...
1600*1/4 = 400
400*1/4 = 100
100*1/4 = 25
For this sequence the common ratio is 1/4. So it doesn't apply
4)
2, 8, 32, 128, ...
2*4 = 8
8*4 = 32
32*4 = 128
as in the 2nd case, this sequence also has a common ratio of 4
5)
16, 12, 8, 4,...
16*4 = 64
the ratio is different to 4
6)
1/2, 1/8, 1/32, 1/128
1/2*4 = 4/2 = 2
1/2*1/4 = 1/8
Again, in this sequence the common ratio is 1/4. So it doesn't apply
From the above, the sequences that have a common ratio of 4, are: 2) and 4)