Trans effect can be defined as the effect of a ligand towards substitution on other ligand which is trans to it.
(weak) F−<HO−<H2O<NH3<py<Cl−<Br−<I−<SCN−<NO−2<SC(NH3)2<Ph−<SO2−3<PR3<AsR3<SR2<H3C−<H−<NO<CO<NC−<C2H4(strong).
Trans effect is nothing but those ligands which are trans directing. That means, which prefer a trans structure. Mostly in Square planar complexes.
Hence option D is correct.