Here's why:

* The 4f subshell is being filled: The presence of electrons in the 4f subshell is the defining characteristic of the lanthanide series.

* Position in the periodic table: Lanthanides are located in the f-block of the periodic table, specifically in the sixth period.

To identify the specific lanthanide element, you would need to consider the atomic number:

* Promethium (Pm) has the electronic configuration 6s² 4f⁵ (although it's often written as 6s² 5d¹ 4f⁴ because of Hund's rule)

* Samarium (Sm) has the electronic configuration 6s² 4f⁶

* Europium (Eu) has the electronic configuration 6s² 4f⁷

Since the given configuration has 4f³ electrons, it represents Samarium (Sm).