Explanation:

- Option 1: between

- Correct choice.

- "Between" indicates the location of the mountain relative to the two towns.

- Used to describe a point that is in the middle or at an equal distance from both towns.

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- Option 2: across

- Not suitable here.

- "Across" would be more useful if referring to a path or a route going from one side of the mountain to the other.

- Option 3: around

- Incorrect choice.

- "Around" implies encircling something, which doesn’t fit the context of being in the middle of two locations.

- Option 4: within

- Doesn’t fit.

- "Within" implies inside a specific area, not applicable as the mountain is not inside one of the towns.

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