Based on the symmetry of the molecules, which is a polar molecule?
secl2 (bent)
becl2 (linear)
bf3 (trigonal planar)
cf4 (tetrahedral)
o2 (diatomic linear)

The molecular structure of the these molecules are shown in the attached document.  SeCl2: Se carries partial "+", Cl carries partial "-", because the molecule is bent, the centers of the "+" and "-" don't overlap, therefore, SeCl2 appears as polar molecule.  The other 4 molecules are all symmetrical, so the centers of the "+" and "-" overlap, therefore these molecules appear not polar.