Which is the most common oxidation state of lanthanides and actinide series?

Q. The common oxidation state of elements of lanthanide series is

Q. Why does iron only have $2+$ and $3+$ oxidation states?

Q. Assertion :Tin exhibits $+2$ and $+4$ oxidation states only and not $+3$ oxidation state. Reason: Inert pair effect of electrons in the s-orbital is responsible.