Write $39$ in roman numbers

To convert $39$ to Roman Numerals we need to split it up into place values (ones, tens, hundreds, etc.):

Place Value    Number       Roman Numeral

Tens                  $30$                    $XXX$

Ones                 $9$                      $IX$

$\Rightarrow$ Please note that we skipped place values that equal $0$.

$39$ can be represented in roman numbers as $XXXIX.$

So option $A$ is the correct answer.