And provide a full explanation you

Let us consider the distance between kensington and greenwich be x
since the given figure is a right angled triangle,we use pythagoras theorem to find the answer

so by pythagoras theorem
x^2= 20^2+40^2
x^2=400+1600
x^2=2000

x=root of 2000=20root 5mi
answer is option (b)