The formation of auroras, also known as the northern and southern lights, is a fascinating natural phenomenon that occurs in the Earth's atmosphere. It is primarily caused by the excitation of certain molecular electrons. The correct answer to the question is option B, which states that the excitation occurs in nitrogen and oxygen molecules.

Auroras occur in the polar regions due to the interaction between the Earth's magnetic field and charged particles from the Sun. When these charged particles, mainly electrons and protons, are ejected from the Sun during solar flares or coronal mass ejections, they are guided towards the Earth's poles by the magnetic field lines.

The excitation of molecular electrons plays a crucial role in the formation of auroras. When the charged particles from the Sun enter the Earth's atmosphere, they collide with the molecules present in the atmosphere, particularly nitrogen and oxygen. These collisions result in the transfer of energy to the molecular electrons, causing them to move to higher energy levels.

The excitation of nitrogen and oxygen molecules leads to the emission of light when the electrons return to their ground state. Nitrogen molecules emit a pinkish-red light, while oxygen molecules emit green or red light, depending on the energy level of the excited electron. These emitted lights form the colorful and mesmerizing displays of auroras in the sky.

The excitation of nitrogen and oxygen molecules is favored because of their abundance in the Earth's atmosphere. Nitrogen comprises about 78% of the atmosphere, while oxygen makes up approximately 21%. This high concentration of nitrogen and oxygen allows for a greater number of collision events and subsequent electron excitation.

Other molecules present in the atmosphere, such as carbon and hydrogen, do not contribute significantly to the formation of auroras. Carbon is present in trace amounts, while hydrogen is primarily found in the form of water vapor. Therefore, the excitation of nitrogen and oxygen molecules is the main source of the vibrant colors seen in the auroras.

In conclusion, the formation of auroras results from the excitation of nitrogen and oxygen molecules in the Earth's atmosphere. The collision between charged particles from the Sun and these molecules leads to the transfer of energy to molecular electrons, causing them to move to higher energy levels. When these electrons return to their ground state, they emit light, creating the stunning displays of the northern and southern lights.