In this question what is the reagent involved in the formation of 1)1-butanol
                                                                                                       3)but-1ene

Q -Write the equations for the preparation of 1-iodobutane from
(i) 1-butanol (ii) 1-chlorobutane (iii) but-1-ene.


(i) 1-Butanol ----->1-iodobutane 
You can use KI with H3PO4
Strong acids  like H2SO4 or HNO3 can not  be used here as they oxidize I- to I2
(iii)
but-1-ene.  ----->1-iodobutane
Electrophilic addition of HBr by anti marks rule which gives   1-bromobutane
now you can do halogen exchange reaction (Finkelstein
reaction.) Use KI in the presence of acetone