Which of these accurately describes inverse variation?

An exponential relationship $y=k{a}^x$, where , which always decreases as $x$ increases

A relationship of the form $y=\frac{k}{x}$ for some constant $k$, with $x\ne 0$

A model where $y$ and $x$ are unrelated but share a common domain restriction of $x\ne 0$

A relationship of the form $y=kx$, where $k$ must be negative