Correct Answer - B
Reactivity order is connected with the order of stability of carbocations :
`1^(@)R^(+) lt 2^(@)R^(+) lt 3^(@)R^(+)`
Both propan-`1-ol` and `2`-methylpropan-`1-ol` are primary alcohols and form `1^(@)` carbocations but latter one forms a branched carbocation which rearranges to form a more stable carbocation :
`CH_(3)CH_(2)CH_(2)OH underset((ii)-H_(2)O)overset((i)H^(+))rarrCH_(3)underset(1^(@)"cation")(CH_(2)overset(o+)CH_(2))underset("shift")overset("hydride")rarrCH_(3)underset(2^(@)"cation")overset(+)(CH)CH_(3)`
`CH_(3)overset(CH_(3))overset(|)(CH)CH_(2)OHunderset((ii)-H_(2)O)overset((i)H^(+))rarr CH_(3)underset(1^(@)"cation")overset(CH_(3))overset(|)(CH)overset(+)(CH_(2)) underset("shift")overset("hydride")rarrCH_(3)underset(3^(@)"cation")underset(+)overset(CH_(3))overset(|)(CH)overset(+)(CH_(2))`
Thus , propan-`1-ol` is least reactive
