The thickness of a paper is 0.004m and that of another paper is 0.008m . Compare their sizes.
$A.$ Paper$ - 1$ is double thicker than paper$ - 2$.
$B.$ Thickness of Paper$ - 1$ is half than paper$ - 2$.
$C.$ Thickness of Paper$ - 1$ is one fourth than paper$ - 2$.
$D.$ Paper$ - 1$ and paper$ - 2$ are both equally thick.

Hint-This question can be solved by checking the options one by one.
Now, given that
 The thickness of paper - 1 is 0.004m
 The thickness of paper - 2 is 0.008m
Now according to option$\left( A \right)$ Paper$ - 1$ is double thicker than paper$ - 2$ but it clear visible that Paper$ - 1$ is not double thicker than paper$ - 2$.Thus, option $\left( A \right)$is incorrect.
Now according to option$\left( C \right)$ thickness of Paper$ - 1$ is one fourth than paper$ - 2$ but it is clearly visible that thickness of Paper$ - 1$ is not one fourth than paper$ - 2$ because the thickness of paper$ - 1$ is $0.004m$ and the thickness of paper$ - 2$ is $0.008m$ and $0.004$ is not one fourth of $0.008$ .Thus option $\left( C \right)$ is incorrect.
Now according to option $\left( D \right)$ paper$ - 1$ and paper$ - 2$ are both equally thick but it is clearly visible that they both are not equally thick. As, $0.004$ and $0.008$ are not equal. Thus the option $\left( D \right)$ is incorrect.
Now according to option $\left( B \right)$ thickness of Paper$ - 1$ is half than paper$ - 2$ and it is clearly visible that thickness of paper$ - 1$ is half than paper$ - 2$. As, $0.004$ is half of $0.008$. Thus the option $\left( B \right)$ is correct.
Hence, the correct option is $\left( B \right)$ .
Note-Whenever we face such types of questions the key concept is that we should check the options one by one and analyze them as we did in this question and in this way we get our answer.