Linear relations are not functions when they appear graphically as:

Vertical lines

linear relations are always functions

horizontal lines

Linear relations are not functions when they appear graphically as:

Vertical lines

One x not one y

Vertical lines is the answer.

A function, by definition, has one domain element mapping to exactly one range element.

If a graph of a linear relation is vertical line, then there is one domain element mapping to infinitely many range elements, which is not a function.

It would also fail the vertical line test because it is a vertical line itself.