Q.Water gas is

Copper does not liberate H2 with dilute HCl (it is below H in the activity series).

A metal displaces hydrogen from a dilute acid only if it lies above hydrogen in the activity/electrochemical series. Zn, Na and Mg are all more reactive than hydrogen and liberate H2. Copper is less reactive (below hydrogen), so it does not react with dilute HCl to give hydrogen gas.

Water gas is the industrial fuel mixture CO + H2, made by reacting steam with red-hot coke.

Water gas is prepared industrially by passing steam over red-hot coke (carbon) at about 1000 degrees C:

C(s) + H2O(g) --(~1000 degrees C)--> CO(g) + H2(g)

This reaction is endothermic, so the coke must periodically be reheated by blowing air through it (which regenerates the heat but also produces some "producer gas", a mixture of CO and N2, as a byproduct of that separate air-blast step). Water gas (CO + H2, roughly in 1:1 ratio) is itself an important industrial fuel and is also the key starting mixture ("synthesis gas") for manufacturing methanol and, via the water-gas shift reaction, additional hydrogen for ammonia synthesis.

The other options don't fit the definition: H2O(g) alone is just steam, CO + N2 describes producer gas (not water gas), and CO + H2O is simply carbon monoxide mixed with unreacted steam, not the defined product.

Zinc, being more reactive than hydrogen, displaces $H_2$ gas when it reacts with dilute sulphuric acid — the standard laboratory method for preparing hydrogen gas.

Step 1 — Recall the reaction:

$Zn(s) + dil.\ H_2SO_4(aq) \longrightarrow ZnSO_4(aq) + H_2(g)\uparrow$

This is a redox displacement reaction: Zn is oxidised ($Zn \to Zn^{2+} + 2e^-$) while $H^+$ ions are reduced ($2H^+ + 2e^- \to H_2$).

Step 2 — Why zinc works but Cu, Hg, Ag don't: In the activity (reactivity) series, metals placed above hydrogen (like Zn, Mg, Fe) can displace $H_2$ from dilute acids because they lose electrons more readily than hydrogen does. Copper, mercury, and silver are all placed below hydrogen in the activity series, so they cannot reduce $H^+$ to $H_2$ and do not react with dilute $H_2SO_4$ to liberate hydrogen.

Water gas is prepared by passing steam over red-hot coke; it is a mixture of carbon monoxide and hydrogen.

Water gas is manufactured industrially by passing steam over red-hot coke at about 1273 K:

C(s) + H2O(g) -> CO(g) + H2(g)

This reaction is endothermic, so the coke bed must be periodically reheated by blowing air through it. The resulting gas mixture, containing roughly equal volumes of CO and H2, is called water gas and is used as a fuel and as a source of hydrogen.

Water gas is CO + H2, made industrially by passing steam over red-hot coke.

Note on syllabus scope: industrial preparation of hydrogen (including water gas) was taught under the Hydrogen chapter of the older NCERT syllabus, which has been dropped from the current rationalised CBSE Class 11 syllabus — flagged out-of-syllabus, answered honestly below.

Water gas is prepared industrially by passing steam over red-hot coke (carbon) at about 1273 K:

C(s) + H2O(g) → CO(g) + H2(g) (this is an endothermic, and also a redox, reaction: carbon is oxidised from 0 to +2, hydrogen is reduced from +1 to 0)

The resulting CO + H2 mixture is called "water gas" and is used as a fuel and as a source of hydrogen (after further reaction with more steam, the "water-gas shift reaction", to convert the CO to CO2 and release more H2).

("Producer gas", by contrast, is CO + N2, made by passing air, not steam, over red-hot coke — that's why option (a) CO + N2 is a distractor for a related but different gas mixture.)