Answer shall be:
D. any triangles
Step-by-step explanation is that:
A non-vertical line is a line that, depending on the slope, either falls from the left to the right, is horizontal (if said slope has value of 0), or falls from the right to the left. To put it simply, think about finding the slope of any line that is not vertical (or does not go directly up and down). Those lines are non-vertical lines.
With that being said, it is quite simple to determine the answer. The question is basically asking what triangles have at least one non-vertical line that we can use to prove the slope. It is impossible to have any sort of triangle that doesn't have at least one non-vertical line; this is because the three sides of a triangle connect to form said shape, and multiple non-vertical lines can only exist if they are parallel to each other (all having a singular, set x-axis value, whilst remaining parallel to the y-axis). Therefore, since there are no triangles that cannot solve this problem, your answer is D) any triangles.
Hopefully this makes enough sense to be of assistance; have a lovely day - stay strong and healthy! :}
