How is the oxidation number related to the group number? If an element $\ce{X}$ forms the highest oxide of the formula $\ce{XO3}$, then it belongs to group:
  A) 14
  B) 15
  C) 16
  D) 17  


How is an oxidation number of an element related to its group?

I tried comparing $\ce{X}$ with $\ce{N}$-, $\ce{S}$-, $\ce{As}$-compounds, but all 3 come in the form $\ce{XO3}$ ($\ce{NO3}$, $\ce{SO3}$, $\ce{AsO3}$), so I can't guess the group number.
For main group elements (groups 1, 2, 13-18), the highest possible oxidation state $O_\text{max}$ is given by the group number $G$ (for groups 1 and 2) or by the group number - 10 (for the rest):
$$ O_\text{max} = \begin{cases} G &amp;\text{if } G \in \{1,2\} \\
G - 10 &amp;\text{if } 13 \leq G \leq 18 \end{cases} $$ 

This means that for your compound $\ce{\stackrel{VI}{X}O3}$, so with the oxidation state VI, the corresponding group would be group 16.