Questions 2 and 3 are testing set phrases : 'for the sake of' and 'tell the difference between' . You can't make set phrases with any of the other options. Question 1 is flawed, in my view, because only 'drink' is incorrect. (It should be 'drinking water'). 'clean' is a possible answer, and 'good' isn't entirely wrong, either. A question such as this wouldn't appear on a 'real' FCE paper.

I disagree with the answer to #1. I think the answer would be "c-fresh" I do think potable would be the best answer since it details two main requirements for water for human consumption: cleaniness and that it be "sweet", not salty. But the question doesn't infer that the water is used for drinking directly. An island by definition is (usually) surrounded by sea water. Water can be cleaned much easier than it can be desalinated. Hence I think the answer would be "fresh"as the idea of fresh water is in contrast to the sea water normally surrounding islands. I do agree with the reasoning for the other two answers. Other words that could be used in those phrases that mean the same thing are: 2- well-being 3- spot

I agree with the concept of "set" phrases for the second two, but ''clean" is the clear choice for the first. Water may be good for washing, but not for drinking. Drinking might be a good choice, but drink is not an adjective, and the sentence requires an adjective. Fresh water may not be potable. The word "potable" is the best choice, but it was not offered. Collocation lists may or may not include these phrases. Learning them comes from extensive reading.

1- a, 2- he remained (with) : c 3- 3 ---- they are kinda tricky lol