Final answer:
Nigeria will be classified as 'water stressed' by 2030.
Step-by-step explanation:
According to the information provided, the prediction for Nigeria's population in 2050 is 402 million. Considering the projected global increase in water stress and the fact that Nigeria is mentioned as one of the areas likely to face a dwindling water supply, it is reasonable to assume that Nigeria will be classified as 'water stressed' by that time. Therefore, the correct answer is 1) 2030.