For the production of ethanol, the most common substrate used in distilleries is

The correct option is Molasses.

Ethanol ($C_2{H}_5\mathrm{OH}$), commonly known as ethyl alcohol, is produced on an industrial scale primarily through the process of microbial fermentation.

During fermentation, micro-organisms like yeast (specifically Saccharomyces cerevisiae) metabolize simple sugars in the absence of oxygen to produce ethanol and carbon dioxide (${\mathrm{CO}}_2$). The general biochemical reaction can be represented as:
$C_6{H}_{12}{O}_6\to 2C_2{H}_5\mathrm{OH}+2{\mathrm{CO}}_2$

For this process to be economically viable in distilleries, a cheap, abundant, and concentrated source of fermentable sugars is required:

1. What is Molasses? Molasses is a dark, viscous by-product obtained during the refining of sugarcane or sugar beets into table sugar. It contains a high concentration (about 50% to 55%) of fermentable sugars, mainly sucrose, glucose, and fructose.

2. Why is it preferred? Unlike starchy substrates like cornmeal or ground gram, which must first undergo enzymatic hydrolysis to break down complex starches into fermentable sugars, molasses already contains readily fermentable simple sugars. This eliminates an extra processing step, making the distillation process highly cost-effective and efficient.

Therefore, molasses is the most common substrate utilized in distilleries for industrial ethanol production.