What polarization has the E-field of the EM wave orthogonal to the E-field of its reference EM wave?

The correct response to the question is based on the concept of polarization in electromagnetic (EM) waves. The term "cross polarization" refers specifically to the situation where the electric field (E-field) of a wave is oriented at a right angle, or orthogonal, to the E-field of a reference wave. In practice, this means that if you have a wave polarized in one direction, a wave that is cross-polarized would be aligned 90 degrees to that direction.

For example, if the reference EM wave has its E-field oriented vertically, a wave with cross polarization would have its E-field oriented horizontally. This characteristic is critical in various applications, including radar and communication systems, where cross-polarization can be utilized to minimize interference and improve signal quality.

Understanding this definition clarifies why the correct answer is specific to cross polarization, distinguishing it from other polarization types such as circular, vertical, or horizontal, which do not inherently imply orthogonality to a reference polarization.