Hall Heroult's process is given by:

First, let us recall what the Hall - Héroult process actually is. It is the industrial electrolytic method used worldwide for extracting aluminium metal from its oxide, alumina. During this process, molten alumina, $$\mathrm{Al_2O_3},$$ dissolved in molten cryolite, is electrolysed. The overall cell reaction that we finally observe is obtained by combining the anodic oxidation of carbon and the cathodic reduction of aluminium ions. Writing it explicitly, we have

$$2\,\mathrm{Al_2O_3\;(l)} + 3\,\mathrm{C\;(s)} \;\longrightarrow\; 4\,\mathrm{Al\;(l)} + 3\,\mathrm{CO_2\;(g)}.$$

This is the universally quoted net reaction for the Hall - Héroult electrolytic reduction of alumina. Hence, while analysing the options, we must look for an equation identical to or equivalent to the above expression.

Now we compare each option one by one.

Option A gives: $$\mathrm{Cr_2O_3 + 2\,Al \rightarrow Al_2O_3 + Cr}.$$ Here chromium(III) oxide is being reduced by aluminium; this is actually an example of the thermite (aluminothermic) process for extracting chromium, not the Hall - Héroult process. So, this option is ruled out.

Option B gives: $$2\,\mathrm{Al_2O_3} + 3\,\mathrm{C} \rightarrow 4\,\mathrm{Al} + 3\,\mathrm{CO_2}.$$ We see that this equation exactly matches the net reaction written above for the Hall - Héroult process. Therefore, this option is fully consistent.

Option C gives: $$\mathrm{Cu^{2+}(aq) + H_2(g) \rightarrow Cu(s) + 2\,H^+(aq)}.$$ This depicts a simple redox displacement of copper(II) ions by hydrogen and has nothing to do with aluminium extraction, so it is not relevant.

Option D gives: $$\mathrm{ZnO + C \xrightarrow{coke,\;1673\;K} Zn + CO}.$$ This equation describes the reduction of zinc oxide by coke in a blast furnace and pertains to zinc metallurgy, again unrelated to the Hall - Héroult process.

From the above detailed comparison, only Option B reproduces the definitive Hall - Héroult overall reaction. Therefore, we choose Option B, which corresponds to Option 2 in the given list.

Hence, the correct answer is Option 2.