The correct answer is: a) Rangpo, Melli.
Industrial estates and zones in Sikkim are located in places like Rangpo and Melli. These places are well-connected to the rest of the country by road and rail. They also have access to water and power supply. In addition, they are located in areas with a good climate and workforce.
Option b) is incorrect because remote villages are not well-connected to the rest of the country and do not have access to basic infrastructure. Option c) is incorrect because high altitude regions are not suitable for industrial development. Option d) is incorrect because border areas are not well-connected to the rest of the country and are prone to security threats.