What is the correct order of ease of replacement of the hydrogen atom by a chlorine atom in the following compound by photochlorination?

Which of the following compounds is least soluble in water at $STP$?

Choose the chain terminating step.
$(1)$ $H_2 \to H^{\bullet} + H^{\bullet}$
$(2)$ $Br_2 \to Br^{\bullet} + Br^{\bullet}$
$(3)$ $Br^{\bullet} + HBr \to H^{\bullet} + Br_2$
$(4)$ $H^{\bullet} + Br_2 \to HBr + Br^{\bullet}$
$(5)$ $Br^{\bullet} + Br^{\bullet} \to Br_2$

Write the reaction for the preparation of ethane from the following:
$(i)$ Ethene
$(ii)$ Ethyl chloride

An alkane $(Y)$ requires $8$ moles of oxygen for complete combustion and on chlorination with $Cl_2/h\nu$, $(Y)$ gives only one monochlorinated product $(Z)$. The total number of primary carbon atoms in $(Y)$ is . . . . . . .

How many primary,secondary and tertiary carbon atoms respectively are present in isobutane?