The function f ( x ) = 200,000 1.21 x gives a company’s predicted annual revenue,

Choice A is correct. It's given that the function $f\left(x\right)=\mathrm{200,000}{\left(1.21\right)}^x$ gives a company's predicted annual revenue, in dollars, $x$ years after the company started selling light bulbs online. It follows that $f\left(x\right)$ represents the company's predicted annual revenue, in dollars, $x$ years after the company started selling light bulbs online. Since the value of $f\left(5\right)$ is the value of $f\left(x\right)$ when $x=5$, it follows that "$f\left(5\right)$ is approximately equal to $\mathrm{518,748}$" means that $f\left(x\right)$ is approximately equal to $\mathrm{518,748}$ when $x=5$. Therefore, the best interpretation of the statement "$f\left(5\right)$ is approximately equal to $\mathrm{518,748}$" in this context is $5$ years after the company started selling light bulbs online, its predicted annual revenue is approximately $\mathrm{518,748}$ dollars.

Choice B is incorrect and may result from conceptual errors.

Choice C is incorrect and may result from conceptual errors.

Choice D is incorrect and may result from conceptual errors.