After understanding the assertion and reason, choose the correct option.
Assertion: In the bonding molecular orbital (MO) of $$H_2$$, electron density is increased between the nuclei.
Reason: The bonding MO is $$\psi_A + \psi_B$$, which shows destructive interference of the combining electron waves.

First, let's understand the Assertion and Reason given in the question.

The Assertion states: In the bonding molecular orbital (MO) of $$H_2$$, electron density is increased between the nuclei.

The Reason states: The bonding MO is $$\psi_A + \psi_B$$, which shows destructive interference of the combining electron waves.

To evaluate these, we need to recall how molecular orbitals are formed in the hydrogen molecule ($$H_2$$). Each hydrogen atom contributes a 1s atomic orbital, denoted as $$\psi_A$$ for atom A and $$\psi_B$$ for atom B. When these atomic orbitals combine, they form two molecular orbitals: a bonding molecular orbital (BMO) and an antibonding molecular orbital (ABMO).

The bonding molecular orbital (BMO) is formed by the constructive interference (in-phase combination) of the atomic orbitals. This is mathematically represented as:

$$\psi_{\text{BMO}} = \psi_A + \psi_B$$

The antibonding molecular orbital (ABMO) is formed by the destructive interference (out-of-phase combination) and is represented as:

$$\psi_{\text{ABMO}} = \psi_A - \psi_B$$

Now, the electron density in a molecular orbital is given by the square of the wave function, $$|\psi|^2$$. Let's compute this for the bonding MO:

$$|\psi_{\text{BMO}}|^2 = (\psi_A + \psi_B)^2 = \psi_A^2 + \psi_B^2 + 2\psi_A\psi_B$$

For the antibonding MO:

$$|\psi_{\text{ABMO}}|^2 = (\psi_A - \psi_B)^2 = \psi_A^2 + \psi_B^2 - 2\psi_A\psi_B$$

In the region between the two nuclei, both $$\psi_A$$ and $$\psi_B$$ are positive because the 1s atomic orbitals are positive everywhere. Therefore, in the bonding MO, the cross term $$2\psi_A\psi_B$$ is positive. This means that the electron density between the nuclei is higher than the sum of the individual atomic orbital densities ($$\psi_A^2 + \psi_B^2$$). Thus, the Assertion is correct: in the bonding MO, electron density is indeed increased between the nuclei.

Now, let's examine the Reason. It claims that the bonding MO is $$\psi_A + \psi_B$$, which is correct, but it states that this represents destructive interference. However, as explained above, $$\psi_A + \psi_B$$ corresponds to constructive interference (in-phase combination), which leads to increased electron density between the nuclei. Destructive interference occurs in the antibonding MO ($$\psi_A - \psi_B$$), where the electron density between the nuclei is reduced. Therefore, the Reason is incorrect because it misattributes destructive interference to the bonding MO.

Given that the Assertion is correct and the Reason is incorrect, we now look at the options:

A. Assertion and Reason are correct, but Reason is not the correct explanation for the Assertion.

B. Assertion and Reason are correct and Reason is the correct explanation for the Assertion.

C. Assertion is incorrect, Reason is correct.

D. Assertion is correct, Reason is incorrect.

Since the Assertion is correct and the Reason is incorrect, the correct choice is D.

Hence, the correct answer is Option D.