The lowest energy structure for a compound would be the one that has the smallest formal charges on its atoms. The formal charge is calculated using the formula:
Formal charge $=$ Total number of valence electrons $-$ Non-bonding electrons $-\frac{1}{2} \times$
Bonding electrons
Thus, for the structure of $SO _{3}$ given in option $A$, the formal charge can be calculated as $=6-4-\frac{1}{2} \times 4=0$
Thus, this structure has the lowest energy.