The correct statements among (a) to (d) regarding $$H_2$$ as a fuel are: (i) It produces less pollutants than petrol. (ii) A cylinder of compressed dihydrogen weighs ~ 30 times more than a petrol tank producing the same amount of energy. (iii) Dihydrogen is stored in tanks of metal alloys like $$NaNi_5$$. (iv) On combustion, values of energy released per gram of liquid dihydrogen and LPG are 50 and 142 kJ, respectively.

We begin by examining statement (i). On combustion, dihydrogen reacts with dioxygen according to the equation $$2\,H_2(g)+O_2(g)\longrightarrow 2\,H_2O(l)+\text{energy}.$$ The only product is water, so no $$CO_2$$, no unburnt hydrocarbons and no soot are formed. Petrol, on the other hand, is a mixture of hydrocarbons and its combustion invariably gives $$CO_2$$, $$CO$$, $$SO_x$$, unburnt hydrocarbons and particulates. Therefore, dihydrogen indeed produces far fewer pollutants than petrol. So, statement (i) is true.

Now we come to statement (ii). The energy released per unit mass of dihydrogen is high, about $$120\;{\rm MJ\,kg^{-1}}$$, but the density of compressed gaseous hydrogen is extremely low. To store a useful amount of hydrogen gas, we need a very strong and therefore very heavy steel cylinder that can withstand high pressure (often >200 atm). NCERT explicitly mentions that “a cylinder of compressed dihydrogen weighs about 30 times as much as a tank of petrol producing the same amount of energy.” Hence the overall weight (cylinder + hydrogen) becomes roughly thirty-fold compared with an equivalent petrol tank. So, statement (ii) is correct.

Next, let us verify statement (iii). One modern method of storing hydrogen is in the interstitial spaces of metal hydride-forming alloys. The typical alloy cited in the textbook is $$NaNi_5$$, which can absorb and release large amounts of hydrogen reversibly: $$NaNi_5 + \dfrac{x}{2}H_2 \rightleftharpoons NaNi_5H_x.$$ Thus, dihydrogen is indeed stored in tanks containing metal alloys such as $$NaNi_5$$. Consequently, statement (iii) is also correct.

Finally, we analyse statement (iv). According to data given in standard references and NCERT, the energy liberated per gram on complete combustion is about $$150\;{\rm kJ\,g^{-1}}$$ for liquid dihydrogen and about $$50\;{\rm kJ\,g^{-1}}$$ for LPG. The statement provided in the question interchanges these two numbers, saying $$50\;{\rm kJ\,g^{-1}}$$ for hydrogen and $$142\;{\rm kJ\,g^{-1}}$$ for LPG. This is the reverse of the correct values, so statement (iv) is false.

Collecting our results, statements (i), (ii) and (iii) are true, while statement (iv) is false. The option that lists exactly (i), (ii) and (iii) is Option D.

Hence, the correct answer is Option D.