What does the notation $f\left(x\right)$ represent in the context of functions relating $x$ and $y$?

A product formed by multiplying the symbol $f$ by the variable $x$, yielding a combined algebraic expression that may represent an area or a compound quantity

The output $y$ corresponding to the input $x$

The instantaneous rate of change of $y$ with respect to $x$ at the point $x$

A new independent variable named $f$, unrelated to the original input $x$, denoting a separate coordinate