What is the simplest construction of a capacitor?

The simplest construction of a capacitor typically involves two metal plates separated by a dielectric material. This configuration is essential for a capacitor to store electrical energy, as the dielectric material increases the capacitor's ability to store charge by reducing the electric field between the plates.

In this context, having two plates allows for the accumulation of opposite charges on each plate, which is fundamental to the function of a capacitor. The dielectric material acts as an insulator, preventing direct current flow between the plates while allowing an electric field to form.

The other configurations, while they relate to capacitive behavior, do not represent the basic structure of a capacitor. A single plate with a dielectric does not store charge in the manner necessary for a capacitor's operation. The option mentioning air as a dielectric does imply capacitance but does not represent the most effective use of materials compared to a dielectric packaged between plates. Lastly, a coil wrapped in plastic would not function as a capacitor in the same way, as it represents inductance rather than capacitance.

Thus, the option describing two metal plates with a dielectric material is the standard and simplest construction of a capacitor, capturing both the necessity of separation and the role of dielectric in charge storage.