The trans-[CoCl2(en)2]+ is optically inactive because it has a plane of symmetry.
The reason is correct. The trans-isomer of [CoCl2(en)2]+ is symmetrical and contains a plane of symmetry, making it optically inactive. In optical isomerism, the lack of symmetry (chiral center) is required, and a plane of symmetry in this case makes the complex achiral (optically inactive).
