What is the smallest $4$ digit number formed using the digits $ 8,3,0,1 $.
$
  A.1038 \\
  B.1308 \\
  C.1083 \\
  D.0138 \\
$

Hint: Write every digit in ascending order and if the $4$ digit number is not formed (means there is no number that can start with $0$) so, replace that number with the smallest number . Use this to find the smallest $4$ digit number.

Complete step-by-step by solution -
According to the question the given numbers are: $8,3,0,1.$
So, to form a $4$ digit number , we can write these number in ascending order
Hence , writing the given number in ascending order : $0 < 1 < 3 < 8$
Since we need a $4$ digit number so it cannot start with $0$, so we can put it in second place.
The smallest digit number other than $0$ is $1$.
$\therefore $the number will begin with $1$.
Thus, the smallest number formed is $1038$.
Hence, the smallest $4$ digit number formed is 1038.

Note: For such types of questions , there is no formula needed to solve such questions, just we can use basic tricks (writing the digits in ascending and descending order) and various methods (hit and trial method or elimination method ) .