Properties of Infrared Waves Steps:

• Statement 1 is false: Infrared waves have wavelengths longer than red light, making them invisible to the human eye.
• Statement 2 is false: Electromagnetic waves travel at exactly 3×10^8 m/s in vacuum; in air, the speed is slightly less (about 99.7% of that value) due to the refractive index.
• Statement 3 is true: Infrared waves are electromagnetic waves, which oscillate perpendicular to their direction of travel, making them transverse.
• Statement 4 is true: Infrared sensors detect heat changes, commonly used in passive intruder alarms to sense motion.

Why D is correct:

• Infrared waves are transverse electromagnetic waves (statement 3) and are utilized in intruder alarms via passive infrared (PIR) detection (statement 4), aligning with electromagnetic wave properties.

Why the others are wrong:

• A includes statement 1, which incorrectly claims visibility.
• B includes statement 2, which inaccurately states the exact speed in air.
• C selects only statement 2, ignoring the correctness of 3 and 4 while including an inaccurate one.

Final answer: D