Answer
The correct fill for the blank provided in the statement is $ a $.
Work Step by Step
In case of limits involving an identity function, for any value of $ a $, as $ x $ gets closer to $ a $, the corresponding value of $ f\left( x \right)$ is just as close to $ a $. That is:
For the identity function $ f\left( x \right)=x $,
$\underset{x\to a}{\mathop{\lim }}\,f\left( x \right)=\underset{x\to a}{\mathop{\lim }}\,\text{ }x=a $.
Where $ a $ is any number.
