Explanation: Thermal noise, also known as Johnson-Nyquist noise, is the primary cause of noise in a RADAR receiver because it is an unavoidable consequence of the random thermal motion of electrons within any electronic component operating above absolute zero. This inherent noise is present in all resistors, transistors, and other conductive materials, generating a wide spectrum of low-level, unpredictable electrical signals. For sensitive RADAR receivers designed to detect extremely faint echoes, this fundamental noise generated within its own components, particularly the front-end amplifier, sets the ultimate limit on how weak a signal can be reliably detected. While other factors like external electrical interference, atmospheric static, or poor grounding can contribute noise, they are either external, mitigatable by design, or less fundamental than the omnipresent thermal noise that defines a receiver's noise floor.