Everywhere you see $x$ in $f(x)=3x-7$, replace it with 4. Write that new expression down before simplifying.

After substituting 4 for $x$, do the multiplication first, then handle the subtraction carefully.

In Desmos, type f(x)=3x-7 to define the function. On the next line, type f(4). Desmos will show a single numerical output next to f(4); that number is the value of $f(4)$.

The expression $f(4)$ means "the value of the function $f$ when $x=4$." Since $f(x)=3x-7$, you find $f(4)$ by replacing $x$ with $4$ in the expression $3x-7$.

Start with the definition of the function:

Now replace $x$ with $4$:

First do the multiplication: $3\cdot 4 = 12$, so

Now subtract:

So the value of $f(4)$ is $5$, which corresponds to choice D.