Oasis is to sand as island is to __________.
Which one of the following options maintains a similar logical relation in the above sentence?

The correct option is Water.

To solve this analogy, we must first analyze the logical relationship between the first pair of words: Oasis and sand.
An oasis is a fertile tract of land containing water, which is completely surrounded by a vast expanse of sand (a desert). Therefore, the defining relationship is: an area of one type of environment completely surrounded by a contrasting medium.

Now, we apply this identical relationship to the second pair: island is to __________.
An island is a piece of land that is completely surrounded by water. Just as an oasis is surrounded by sand, an island is surrounded by water. Therefore, the word "water" maintains the same logical connection.

We can also verify why the other choices are incorrect:
- Stone: Islands are not defined as being surrounded by stone.
- Land: An island is made of land, so it cannot be surrounded by land; a piece of land surrounded by land is simply part of the mainland.
- Mountain: An island is not surrounded by mountains; rather, it is surrounded by a body of water.