Which material reflects RADAR beams less effectively?

Grass reflects RADAR beams less effectively compared to materials such as concrete, metal, or stone. This is primarily due to the texture and composition of grass. Grass is composed of organic material and has a relatively low density, which contributes to its ability to absorb rather than reflect RADAR waves. Unlike more solid and dense materials such as concrete or metal that have smooth surfaces capable of reflecting signals effectively, grass has a more irregular and softer surface which scatters the RADAR waves rather than reflecting them back to the source.

The effectiveness of RADAR reflection is influenced by the surface area and material properties, with hard and shiny surfaces, like those found in metals and concrete, being more efficient in bouncing back RADAR signals. Consequently, when trying to detect movement or objects, grass would be less reliable compared to these other materials, which explain why it is identified as the correct answer.