The fundamental principle of wireless coexistence is managing the shared radio frequency (RF) spectrum to prevent interference. Different wireless solutions operating in the same physical area must use the RF spectrum in a coordinated manner to avoid degrading each other's performance. Coordinating frequency use, also known as channel planning, ensures that transmissions from one system do not conflict with another, which is the most critical factor for successful coexistence. This involves selecting non-overlapping channels and managing power levels to minimize co-channel and adjacent-channel interference.

A. Coordinating mounting locations with the other wireless solutions: While physical separation can help reduce interference, it is secondary to proper frequency coordination. Two systems can be close together and coexist perfectly if on different frequencies, but interfere even when far apart if using the same conflicting frequency.

B. Implementing matching encryption keys synchronized with the other wireless solutions: Encryption keys are a security mechanism for data confidentiality and have no role in managing RF transmissions or preventing physical layer interference. Sharing keys between disparate systems is also a significant security vulnerability.

C. Implementing contrasting encryption keys countering the other wireless solutions: This is incorrect for the same reason as option B. Encryption keys are unrelated to RF coexistence. The use of different (contrasting) keys is standard for security but does not affect how the systems interact in the RF spectrum.

1. IEEE Communications Surveys & Tutorials: In a comprehensive survey on wireless coexistence, the authors define the core problem: "The problem of coexistence arises when two or more otherwise independent wireless systems operate in the same frequency band and in close physical proximity." This establishes that managing frequency use is the central issue.

Source: Vanlin Sathya, et al. "A Survey of Wireless Coexistence: Challenges, Solutions, and Opportunities." IEEE Communications Surveys & Tutorials, vol. 18, no. 1, Firstquarter 2016, pp. 22-46, Section I (Introduction). DOI: 10.1109/COMST.2015.2471153.

2. University Courseware (Carnegie Mellon University): Course materials on wireless networks emphasize that the physical layer's primary challenge is sharing the wireless medium (spectrum). Techniques like Frequency Division Multiple Access (FDMA), which is the basis for channel planning, are presented as fundamental methods to allow multiple systems to operate simultaneously without interference.

Source: Ganti, R. K. (2012). 18-759: Wireless Networks, Lecture 3: Wireless Physical Layer. Carnegie Mellon University, Department of Electrical and Computer Engineering. Slide 12, "Multiple Access".

3. University Courseware (Stanford University): In lectures on wireless communication, the concept of interference as the limiting factor in wireless network capacity is a core topic. The necessity of orthogonal resource allocation (such as using different frequency channels) is highlighted as the primary method to mitigate this interference and enable coexistence.

Source: Goldsmith, A. (2009). EE359: Wireless Communications, Lecture 1: Introduction to Wireless Communications. Stanford University. Section on "Challenges in Wireless Communications".