Answer
$O=C=O$
Work Step by Step
Possible resonance structures:
$O=C=O$
$^-O-C\equiv O^+$
$^+O\equiv C-O^-$
The best resonance structure for CO2 from a formal charge standpoint is the one in which both oxygen atoms have a formal charge of 0. This is achieved by having a double bond between the carbon and each oxygen atom, and it is the most stable resonance:
$O=C=O$
