Where does the last electron of d-block element go?

In lanthanides, Ce changes from +4 to +3 oxidation state.

Which of the following statement is correct?

Which of the following lanthanide ions do not exhibit color?

The study of transition elements is useful.

Actinoids are mostly attacked by which acid?

What is the common oxidation state of scandium? (At. No. of Sc = 21)

Which of the following is the correct order of arrangement of the first five lanthanides according to atomic number?