Which of the following is the correct answer of ${\left(998\right)}^2$?

Given that ${(998)}^2.$ 

We can write 998 as $\left(1000-2\right).$  
We will use the formula ${\left(\mathrm{a}-\mathrm{b}\right)}^2={\mathrm{a}}^2-2\mathrm{ab}+{\mathrm{b}}^2$  to solve the given question and simplify our calculations.

$\therefore {998}^2={\left(1000-2\right)}^2$ 
Now solving using the above formula and taking $\mathrm{a}=1000\text{ and }\mathrm{b}=2,$ we get,
$\begin{array}{l}\therefore {998}^2={\left(1000-2\right)}^2\end{array}$

$\begin{array}{l}\Rightarrow {998}^2={1000}^2+2^2-2\times 1000\times 2 \end{array}$

$\begin{array}{l}\Rightarrow {998}^2=1000000+4-4000\\ \Rightarrow {998}^2=996004\end{array}$  

Therefore, 996004 is the correct answer.