The element californium belongs to a family of :

The correct option is Actinide series.

To understand why this is the correct classification, let us look at the structure of the periodic table step-by-step:

1. What is Californium?
Californium is a synthetic, radioactive chemical element with the chemical symbol Cf and atomic number 98.

2. Periodic Table Organization:
The periodic table is arranged by atomic numbers. The elements with atomic numbers ranging from 89 to 103 are grouped together at the bottom of the periodic table in a specific row. This group of elements is known as the Actinide series (or actinides).

3. Placement of Californium:
Since the atomic number of Californium is 98, which falls directly within the range of $89\le Z\le 103$ (where $Z$ represents the atomic number), it belongs to the Actinide series. All elements in this series are f-block transition metals and are characterized by the filling of the 5f electron shell.