The set in which compounds have different nature is:

We need to identify the set in which the compounds have different chemical natures from each other.

Option 1: B(OH)$$_3$$ (boric acid) is a Lewis acid (acidic), and H$$_3$$PO$$_3$$ (phosphorous acid) is also acidic. Both are acids, so they have the same nature.

Option 2: B(OH)$$_3$$ is acidic (it acts as a Lewis acid, accepting electron pairs from water). Al(OH)$$_3$$ is amphoteric — it can act as both an acid and a base. Since B(OH)$$_3$$ is purely acidic and Al(OH)$$_3$$ is amphoteric, they have different natures.

Option 3: NaOH is a strong base and Ca(OH)$$_2$$ is also a base (though slightly less soluble). Both are basic in nature, so they have the same nature.

Option 4: Be(OH)$$_2$$ is amphoteric and Al(OH)$$_3$$ is also amphoteric. Both have the same nature.

The set where the compounds have different natures is option 2: B(OH)$$_3$$ (acidic) and Al(OH)$$_3$$ (amphoteric).