Answer: 8 x 10^5 which is choice A
Explanation
Place the decimal point between the first two digits. Then ignore the rest of the digits.
We go from 800,000 to 8.0
The question is now: how do we go from 8.0 back to 800,000?
We'll need to move the decimal point 5 spots to the right.
That explains why the exponent over the 10 is 5.
Therefore 800,000 = 8.0 x 10^5
Since 8.0 is the same as 8 (assuming we aren't worried about sig figs of course), this means 8.0 x 10^5 is the same as 8 x 10^5
Notice 10^5 = 100,000 = hundred thousand
So 8 x 10^5 means "8 times hundred thousand" aka "8 hundred thousand" aka "800 thousand".