Since AB and AC are on the same line, their slopes are the same, eliminating A.
Use rise/run to find the slope for AC. 6/9 = 2/3
Option B is eliminated.
Option C is incorrect, because as mentioned earlier, they're on the same line, which means they have the same slope.
Option D is correct because 2/3 = 4/6
Therefore the option that is not true would be C.