Explanation:

Population size is a key demographic characteristic of states in India. The question asks to identify the state with the smallest population among the given options. \nOption A, 'Sikkim', is the correct answer. According to the $2011$ Census, Sikkim has the smallest population among all states in India, with a total population of approximately $6.1$ lakh ($610,577$). Its mountainous terrain and relatively small area contribute to its low population density. \nOption B, 'Goa', is incorrect. While Goa is the smallest state by area, its population, as per the $2011$ Census, was approximately $14.5$ lakh ($1,458,545$), which is significantly higher than Sikkim's population. \nOption C, 'Manipur', is incorrect. Manipur's population, according to the $2011$ Census, was approximately $28.5$ lakh ($2,855,794$), making it more populated than both Sikkim and Goa. \nOption D, 'Mizoram', is incorrect. Mizoram's population, as per the $2011$ Census, was approximately $11.2$ lakh ($1,120,797$), which is also higher than Sikkim's population.