First we need to find "d" which is difference between any 2 successive members of the sequence. Because the sequence is arithmetic that means that the difference between any 2 successive members is same and constant.
This is the formula for any member of sequence at position "n"
An = A1 + (n-1)*d
d = (a9 - a1)/(9-1)
d = 5
Now we calculate 25th member:
a25 = a1 + (25-1)*5
a25 = 128
Answer is d)
