To compare the given numbers, we need to make sure they are written in the same form. Let's rewrite them in scientific notation:

a) 4.25 * 10^0 = 4.25
b) 8.9 * 10^(-2) = 0.089
c) 7.9 * 10^2 = 790
d) 6.34 * 10^(-1) = 0.634

Now we can compare them:

a) 4.25 < 0.089 (False)
b) 0.089 < 790 (True)
c) 790 < 0.634 (False)
d) 0.634 < 790 (True)

Therefore, the correct comparison is:

b < d