Answer:
==> Presence of unpaired electrons.
==> Electrons in ns and electrons in (n-1)d orbitals are used to form bonds because of their similarities in energies.
Step-by-step explanation:
The d- metals are also known as the transition metals with metal elements such as the ones given in the question, that is; Copper, iron and Manganese. Others include chromium, Nickel, Cobalt and so on.
The reason behind many d-metals forming cations with different oxidation states is mainly because of the unpaired electrons. Because Unpaired electrons are unstable they tends to attract or bond with other species chemically.
Also, many d-metals form cations with different oxidation states because Electrons in ns and electrons in (n-1)d orbitals are used to form bonds because of their similarities in energies.