This structure has a plane of symmetry that passes through C3 and its geminal hydrogens. The chiral carbon to the left of the plane is of the (S)-configuration while the one to right of the plane is of the (R)-configuration. Clearly, this conformation of the molecule with its zig-zag carbon chain is achiral. This CONFORMATION DOES NOT contribute to optical activity.
Click here for help with R and S configuration.