Explanation: RADAR receivers are designed to detect targets by analyzing reflected radio waves. However, the environment itself can reflect these waves, generating unwanted echoes known as "clutter." The most common types of natural clutter are **weather** and **sea return**. **Weather** refers to precipitation like rain, snow, or hail, which can reflect radar pulses strongly, creating widespread echoes that obscure true targets. **Sea return** occurs when radar pulses reflect off ocean waves, generating a pattern of echoes that can mask vessels or other objects on the water's surface. These phenomena are prevalent and significantly impact target detection, making them the most common sources of interference. While thermal noise is always present in electronics, and electrical noise can occur, they are generally less dominant than environmental clutter in masking targets. Jamming is deliberate interference, not a common natural occurrence.