Barrons AP Calculus - David Bock

(D) Of the four remaining slope fields, IV is the only one whose slopes are not equal along either
a vertical or a horizontal line (the segments are not parallel). Its d.e. therefore cannot be either of
type y ′ = f (x) or y ′ = g(y). The d.e. must be implicitly defined—that is, of the form y ′ = F(x, y).
So the answer here is IV.