The technology used to ward off insects and harmful elements from homes is; option [A] Ultrasonic wave. Ultrasonic pest repellent devices emit high-frequency sound waves (typically above 20,000 Hz) that are inaudible to humans but irritating to pests such as insects, rodents, and spiders. These devices are designed to deter pests from entering or residing in the treated area by creating an uncomfortable environment through sound.

Ultrasonic pest control is favored for its non-toxic nature compared to traditional chemical methods. It’s commonly used in homes, offices, and other indoor spaces where pests are unwelcome. However, effectiveness can vary depending on factors like the type of pest, room layout, and frequency used. While ultrasonic waves can deter pests, they do not typically eliminate infestations already present and may require additional pest management strategies for comprehensive control.

Thus, the correct answer for technology used to repel insects and harmful elements from homes is [A] Ultrasonic wave.