Which of the following is the best definition of vertical angles? Vertical angles are angles formed by two intersecting lines.
Vertical angles are formed by intersecting a vertical and horizontal line.
Vertical angles are a pair of non-adjacent angles formed by two intersecting lines.
Vertical angles are a pair of congruent angles with a vertical line of symmetry.

Step-by-step explanation:

Very quick and effortless example of vertical angles in the attached image. When 2 straight lines intersect, the 2 angles opposite each other at that point are vertical angles, and they are always congruent.

I'd say it's the 3rd option:

"Vertical angles are a pair of non-adjacent angles formed by two intersecting lines."

but any of the first three could be technically true really. Adjacent angles are 2 angles that share a side, and vertical angles cannot share one.

for ex: the reciprocal of 3/4 is 4/3. it is the opposite.

reciprocal definition:

a mathematical expression or function so related to another that their product is one; the quantity obtained by dividing the number one by a given quantity.