The correct statement(s) about transition elements is/ are :

The transition element's most stable oxidation state is +3, and its stability decreases over time. From Cr to Cu, the transition elements of the 3d-series have almost identical atomic sizes. With increasing atomic number, the +2 state becomes more stable in the first half of the first row transition elements as 3d orbitals acquire only one electron in each of five 3d orbitals (remain half-filled) and the electronic repulsion is least and the nuclear charge increases. Some transition elements, such as Ni, Fe, and Cr, may have zero oxidation states in some of their compounds.