In the industrial production of which of the following, molecular hydrogen is obtained as a byproduct ?

Options

A. NaOH
B. NaCl
C. Na metal
D. `Na_2 CO_3`

Explanation

To determine in the industrial production of which compound molecular hydrogen is obtained as a byproduct, we can analyze the process involved in the production of sodium hydroxide (NaOH) through the electrolysis of brine (a concentrated solution of sodium chloride, NaCl).

### Step-by-Step Solution:

1. **Identify the Process**: The industrial production of sodium hydroxide (NaOH) is commonly achieved through the electrolysis of brine (NaCl solution).

2. **Electrolysis Reaction**: During electrolysis, the brine solution undergoes the following reactions:
- At the cathode (negative electrode), sodium ions (Na⁺) are reduced to form sodium amalgam (Na-Hg):
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