Which of the following is a natural source of electromagnetic interference (EMI)?

Lightning is a natural source of electromagnetic interference (EMI) because it produces a significant amount of electromagnetic energy during its discharge. When lightning strikes, it generates a strong and wide-spectrum electromagnetic field that can disrupt electronic devices over a considerable distance. This phenomenon can cause interference in radio communications, affect navigational aids, and impact any sensitive electronic equipment nearby.

In contrast, power lines, Wi-Fi routers, and radio transmitters are man-made sources of EMI. While they can also produce electromagnetic interference, they do not originate from natural occurrences. Power lines emit electromagnetic fields due to the flow of electrical current, Wi-Fi routers utilize radio frequencies for wireless communication, and radio transmitters broadcast signals for communication purposes—all of which are human-designed systems. Therefore, the distinctive characteristic of lightning being a natural occurrence is what makes it the correct answer in identifying a natural source of EMI.