What is the simplest form of a practical antenna that is very short compared with the wavelength of operating frequency?

The simplest form of a practical antenna that is very short compared with the wavelength of the operating frequency is indeed a doublet. A doublet antenna, often referred to as a dipole antenna, consists of two conductive elements (wires or rods) that are equal in length and are oriented in a straight line, making it effectively a basic form of dipole.

When working with antennas that are much shorter than the wavelength, a doublet provides a reasonable balance of simplicity and effectiveness in radiating or receiving signals. It's relatively easy to construct and deploy, using basic materials, which makes it accessible for various applications. The characteristics of the doublet allow it to function fairly well across a range of frequencies, even though it may not be as effective as larger antennas optimized for specific wavelengths.

In contrast, other options like collinear, rhombic, and directional antennas typically involve more complexity in design, construction, or orientation. These antennas may offer specific advantages in certain situations, such as improved gain or directivity, but they are not as straightforward or practical when dealing with very short antennas relative to the wavelength. Thus, the doublet emerges as the most straightforward choice for applications involving shorter antennas.