To solve this problem, we need to match each item in column (a)-(d) with the correct description in column (i)-(iv). Let's analyze each one:$\\$(a) CO(g) + H$_2$(g):$\\$This is known as synthesis gas or syngas, which is a mixture of carbon monoxide and hydrogen.$\\$Thus, (a) matches with (iii) Synthesis gas.$\\$(b) Temporary hardness of water:$\\$Temporary hardness is caused by the presence of bicarbonates of calcium and magnesium,$\\$which can be removed by boiling. Therefore, (b) matches with (i) $$Mg(HCO$_3)_2 +$Ca(HCO$_3)_2. \\$(c) B$_2$H$_6: \\$Diborane $(B_2H_6)$is an electron-deficient compound because it does not have enough electrons$\\$to form conventional covalent bonds. Thus, (c) matches with (ii) An electron deficient hydride.$\\$(d) H$_2$O$_2: \\$Hydrogen peroxide $(H_2O_2)$ has a non-planar structure due to the presence of lone pairs on the oxygen atoms.$\\$Therefore, (d) matches with (iv) Non-planar structure.$\\$Based on the analysis, the correct matches are:$\\$(a) -> (iii), (b) -> (i), (c) -> (ii), (d) -> (iv)$\\$This corresponds to Option 4.