Final answer:
The lowest possible frequency of an emission line when the final energy shell is always 1 would fall under the category of radio waves, which are at the lowest frequency end of the electromagnetic spectrum.
Step-by-step explanation:
The lowest possible frequency of an emission line when the final energy shell is always 1 would be in the category of radio waves. This is because radio waves are at the lowest frequency end of the electromagnetic spectrum. As electrons drop to the first energy level, they release energy in the form of electromagnetic radiation. A transition to the first shell from higher shells would result in higher energy, and therefore high-frequency photons, not low-frequency radio waves. However, when considering the entire spectrum of electromagnetic radiation and the types listed in the options (infrared, ultraviolet, microwave, and radio), radio waves are generally the lowest frequency electromagnetic waves, which includes everything lower in frequency than visible light.