Which of the following is not an emerging technology?

An emerging technology is one that is currently developing or will have significant impact in the near future. Artificial Intelligence, quantum cryptography, and 3-D holographic imaging are all active areas of research with ongoing advancements, making them emerging technologies. In contrast, computers based on vacuum tubes were the first generation of electronic computers used in the 1940s and 1950s. They have been completely replaced by transistors and integrated circuits due to their large size, high power consumption, and unreliability. Since vacuum tube technology is obsolete and no longer under development for modern computing purposes, it does not qualify as an emerging technology. Therefore, the correct answer is Option D.