Boiling point $$\propto$$ Molecular weight
Boiling point $$\propto$$ Branching.
Of n-pentane, n-hexane and 2,3-dimethyl butane. n-pentane has the lowest mass and 2,3-dimethyl butane has the highest mass same as that of n-hexane but 2,3-dimethyl butane has higher branching than n-hexane.
So, the order of boiling point is
2,3-dimethyl butane > n-hexane > n-pentane
Hence, hydrocarbon with the highest boiling point is 2,3-dimethyl butane and that with the lowest boiling point is n-pentane.