Your answer is A) 18 units
(1,3),(7,3),(7,7), and (4,7)?
(1,3) ---> (7,3)
You remained at height y = 3, and went from x = 1 to x = 7, so horizontally you went 6 to the right.
(7,3)----->(7,7)
x = 7 in the first and x = 7 in the second, you didn't walk anything in the x direction, your height y changed from 3 to 7, so you went vertically up 4.
(7,7) --> (4,7)
You height remained the same, you went from 7 to 4 in the x direction, that's 3 to the left.
Finally, to get back to point (1,3) - (4,7) ----> (1,3), 3 to the left and 4 down, the diagonal being 5.
6+4+3+5= 10 + 8 = 18.