The correct answer is: a) English.
English is the medium of instruction in most Sikkimese schools. This is because Sikkim is a multilingual state with no single majority language. English is seen as a neutral language that can be used to communicate with people from different language backgrounds.
Nepali is the second most widely spoken language in Sikkim, followed by Hindi. However, these languages are not used as the medium of instruction in most schools. This is because they are not seen as neutral languages, and they can be difficult for students from other language backgrounds to learn.
A mix of languages is not used as the medium of instruction in most Sikkimese schools. This is because it would be difficult for teachers to teach in a mix of languages, and it would be difficult for students to learn in a mix of languages.
In addition to English, some schools in Sikkim also offer instruction in Nepali or Hindi. This is done to accommodate students who are not fluent in English. However, English is still the main medium of instruction in most schools.