The circle represent the earth .the radius of earth is about 6400km. how do i find the distance d given the height

(This is assuming d=diameter, the way you phrased this was a bit vague)
Well, with circlesthere is no such thing as "Height"
What you were given, and all you need to find the diameter is the radius
Diameter=radius*2
Radius=Diameter/2
Well, you need to find the diameter, so let's go with the first choice, We already know that the radius of the earth is 6400 km
So, you would simply take that times 2 to find the diameter.
6400*2= 12,800 km
The answer is D=12800 KM
(Once again, this is assuming you mistakenly said height, and d is equal to diameter, though I highly believe you mis-phrased this question and meant something else)


So, I took the liberty of researching the ENTIRE question, as it was supposed to be put, and I believe I found it, though it is worded very differently than the way you put it:
(6412)^2=6400^2+D^2
41113744-40960000=d^2
153744=d^2
392.1 km=D
This is the correct answer assuming THIS is what you meant: The circle at the right represents Earth. The radius of the h. Earth is about 6400 km. Find the distance d that a person can see on a clear day from each of the following heights h above earth. 
~Hope this helps!