Assertion: The element with electronic configuration 4s2 3d2 is a d-block element.
Reason: The last electron enters the d orbital.

Explanation:

The electronic configuration of an element is a representation of the arrangement of electrons in its atomic orbitals. The electronic configuration is written using a standard notation that includes the number of electrons in each energy level and the type of orbital they occupy.

1. Electronic configuration notation:
The electronic configuration notation follows the pattern:
1s2 2s2 2p6 3s2 3p6 4s2 3d2

2. Identification of the valence electrons:
The valence electrons are the electrons in the outermost electron shell, also known as the highest energy level. In this case, the valence electrons are in the 4s and 3d orbitals.

3. Explanation of the assertion:
The assertion states that the element with the electronic configuration 4s2 3d2 is a d-block element. This assertion is correct.

- The d-block elements are located in the middle of the periodic table, between the s-block and p-block elements.
- The d-block elements have their last electron(s) filling the d orbitals.
- In this case, the last two electrons fill the 4s orbital, and the next two electrons fill the 3d orbital.
- Therefore, the element with the electronic configuration 4s2 3d2 is a d-block element because the last electron enters the d orbital.

In conclusion, the reason given in the assertion is valid. The element with the electronic configuration 4s2 3d2 is a d-block element because the last electron enters the d orbital.