$\text{ What is the oxidation number of Mo in }{\mathrm{MoO}}_2{\mathrm{Cl}}_2\text{ ? }$

Molybdenum must have a positive charge, but if the complex as a whole has a charge of -2, there are still 2 remaining negative charges.

$x - 8 = -2$

$x=-2+8$

$x=+6$

As a result, the oxidation number of Mo in $Mo{O}_{2}C{l}_{2}is+6.$

Hence, option D is correct.