Which of the following oxidation states is common for all lanthanides?

Which element from the following has a completely filled $f$-orbital in both observed and expected electronic configurations?

Which statement is incorrect for the electronic configuration $[Xe]\, 4f^{7}\, 5d^{1}\, 6s^{2}$?

Choose the correct option regarding the following.
Assertion: Actinides exhibit a larger number of oxidation states than lanthanides.
Reason: There is a large energy gap between $5f$,$6d$ and $7s$ sub-shells.

The elements in which electrons are progressively filled in $4f$ orbital are called

The common stable oxidation states of $Eu$ and $Gd$ are respectively