Explanation:

- Actinium: While part of the actinide series, which fits within the f-block elements, it is not a lanthanide.

- Francium: An alkali metal, not related to the lanthanides.

- Cerium: This is the correct answer. Cerium is indeed a lanthanide, part of the f-block elements, specifically placed in the lanthanide series which includes atomic numbers 57 through 71.

- Polonium: A metalloid, part of the p-block, and not associated with the lanthanide series.