D-Block Elements are characterized by the filling of inner d orbitals by electrons.

The d-Block Elements, also known as transition metals, are a group of elements located in the middle of the periodic table between the s-Block and p-Block elements. These elements have partially filled d orbitals in their electron configuration, which gives them unique properties and characteristics.

Here is a detailed explanation of why option 'B' is the correct answer:

1. Definition of d-Block Elements:
- The d-Block Elements are the elements that have their outermost electron in one of the d orbitals. These elements include the groups 3 to 12 in the periodic table.
- The d orbitals are the five orbitals with the azimuthal quantum number 'l' equal to 2. They are designated as dxy, dxz, dyz, dx^2-y^2, and dz^2.

2. Electron Configuration of d-Block Elements:
- The electron configuration of d-Block Elements involves the filling of the 3d and 4s orbitals.
- The 3d orbitals have a higher energy level than the 4s orbital, so the 4s orbital is filled before the 3d orbitals.
- The electron configuration of d-Block Elements follows the general pattern of [noble gas] (n-1)d^1-10 ns^1-2, where n represents the principal quantum number of the period.

3. Filling of Inner d Orbitals:
- The key characteristic of d-Block Elements is the filling of the inner d orbitals by electrons.
- As the atomic number increases, electrons are added to the 3d orbitals until they are completely filled.
- The filling of the 3d orbitals is responsible for the transition metals' unique properties, such as variable oxidation states, formation of colored compounds, and the ability to act as catalysts.

4. Examples of d-Block Elements:
- Some examples of d-Block Elements include iron (Fe), copper (Cu), zinc (Zn), silver (Ag), and gold (Au).
- These elements exhibit a wide range of properties due to the partially filled d orbitals, such as high melting and boiling points, good conductivity, and the ability to form complex ions.

In conclusion, d-Block Elements are characterized by the filling of inner d orbitals by electrons. This characteristic gives these elements their unique properties and distinguishes them from other elements in the periodic table.