Which single sentence most precisely defines a one-to-one function (injective)?

Which simple graphical procedure determines whether a curve $y=g\left(x\right)$ represents a function of $x$?

A one-to-one function $f$ is defined on integers by $f(-2)=9$, $f\left(0\right)=3$, $f\left(2\right)=7$, $f\left(5\right)=1$. Create $f^{-1}$ explicitly and use it to compute $f^{-1}\left(7\right)$.

A graph consists of the parabola $y=x^2$ and the line $y=2$. Using vertical and horizontal line tests, determine whether the parabola is a function, whether it is one-to-one, and whether its restriction x ≥ 0 is one-to-one. Provide justification.

A function $f$ maps temperatures in Celsius to voltages measured by a sensor: $f=\left\{\right(0,0.00),(25,0.50),(50,1.05),(75,1.65\left)\right\}$. The inverse function $f^{-1}$ is used to convert voltage back to temperature. What problem occurs when trying to evaluate $f^{-1}\left(2.00\right)$?