The general outer electronic configuration of f-block elements can be represented as
$
\left( A \right)\left( {n - 2} \right){f^{1 - 14}}\left( {n - 1} \right){d^{0 - 1}}n{s^2} \\
\left( B \right)\left( {n - 2} \right){f^{1 - 14}}\left( {n - 1} \right){d^{0 - 5}}n{s^{0 - 2}} \\
\left( C \right)\left( {n - 2} \right){f^{1 - 14}}\left( {n - 1} \right){d^{0 - 10}}n{s^{1 - 2}} \\
\left( D \right)\left( {n - 2} \right){f^{1 - 14}}\left( {n - 1} \right){d^{0 - 2}}\left( {n - 1} \right){s^2} \\
 $

Hint: In f-block elements, the differentiating electron enters the f-orbital of the antepenultimate shell. That is, while filling electrons, the last electron enters the f-orbital of the shell which is two shells before the valence shell.

Complete answer:
When we write a general electronic configuration, the valence shell which is the outermost shell is written as “n”. One shell before the valence shell is known as the penultimate shell. It is represented as (n-1). The shell which is present before the penultimate shell is called an anti penultimate shell and it is represented as (n-2). In f-block elements, the differentiating electrons enter the f-orbital of the anti-penultimate shell. Also, the maximum number of electrons that can be accommodated in the f-orbital is 14. Hence, their general outer electronic configuration begins with $\left( {n - 2} \right){f^{1 - 14}}$ . Another aspect we can look into is that the electronic configurations of all f-block elements end with 2 electrons in the s-orbital of the valence shell. That is, the general electronic configuration ends with $n{s^2}$ . Among the given options, the beginning and the end of the general outer electronic configuration is mentioned only in option (A) and hence that is the right answer. That is, the correct general outer electronic configuration of f-block elements is $\left( {n - 2} \right){f^{1 - 14}}\left( {n - 1} \right){d^{0 - 1}}n{s^2}$ .

Hence the correct answer is A).

Note: f-block elements are the two rows of elements that are placed separately at the bottom of the periodic table. The series which begins with the element lanthanum is called lanthanide series and the row which begins with the element actinium is known as actinide series.