What form of energy emission accompanies the return of excited electrons to the ground state?

When electrons are returns from excited state to ground state it emits energy in the form of electromagnetic radiation or electromagnetic wave.

Electromagnetic radiation is made when an atom absorbs energy. The absorbed energy causes excitation of electron from ground state to higher energy levels. The excited electron returns to its original position by the emission of electromagnetic radiation.

Some properties of electromagnetic radiations are,

Thus, we can conclude that, when electrons are returns from excited state to ground state it emits energy in the form of electromagnetic radiation or electromagnetic waves.

Learn more about electromagnetic radiation here:

Electromagnetic radiation

Explanation:

When an excited electron returns to the ground state, it releases the excess energy as a quantum of electromagnetic radiation.

The wavelength of the radiation can vary from about 100 nm (the ultraviolet region) to about 10 000 nm (the infrared region).

A small slice of this range (about 400 nm to 800 nm) is the visible region, where our eyes can detect the radiation as different colours of light.

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answer is: c. the experiment shows that the red substance experienced a chemical change.

this is chemical change (chemical reaction), because two new substances are formed (metalig liquid and gas), the atoms are rearranged and the reaction is followed by an energy change.

this is example of chemical decomposition.

chemical decomposition (analysis) is the separation of a single chemical compound into its two or more elemental parts or to simpler compounds.  

chemical decomposition is opposite of chemical synthesis.