In mathematics, a percentage is a number or ratio expressed as a fraction of 100.
Electric field due to a point charge is given as

here we know that

also the distance is given as

now we will have

so we will have

so above is the electric field due to proton
Iron oxide = small region within a magnet
drop or hammer =man-made magnet
strength decreases rapidly with distance lines of force
The answer is b. The crest.
Hope this helps!