In the past four days Ken runs 20 miles.
We can find this as the sum of the miles of the four days:

If he runs 10% less in the next four days. We assume that each day he will run 10% of the corresponding day (5th day compared to 1st days, 6th day compared to 2nd day, and so on).
Then, the 8th day he will run 10% less than 8 miles, what he run on the 4th day:

Then, the difference of what he ran in the 8th day compared to the 4th day is:

Answer: The closest option is Option A (0.9 miles)