In the context of alternative sources of energy, ethanol as a viable bio-fuel can be obtained from various biomass feedstocks, including the options you provided: potato, rice, sugarcane, and wheat. However, the most commonly used and efficient feedstock for ethanol production is sugarcane (Option 3).

1. Potato: While potatoes can be used to produce ethanol, they are not the preferred feedstock due to several factors. Firstly, potatoes have a lower starch content compared to other crops like corn and wheat, which makes their conversion into ethanol less efficient. Additionally, potatoes are a staple food crop and are widely consumed worldwide, so diverting them to ethanol production may have negative implications for food security.

2. Rice: Rice can also be used as a feedstock for ethanol production. However, rice is a staple food for a large portion of the global population, particularly in Asia, and using it for biofuel production may lead to competition between food and fuel production. This can potentially impact food prices and availability. Consequently, using rice as a biofuel feedstock is often considered less desirable.

3. Sugarcane: Sugarcane is one of the most suitable and widely used feedstocks for ethanol production. It contains a high concentration of sucrose, which can be converted into ethanol through fermentation. Brazil, for example, has successfully implemented large-scale sugarcane-based ethanol production, making it one of the leading countries in the biofuel industry. Sugarcane offers several advantages, including high ethanol yield per unit of land, efficient conversion processes, and potential co-generation of electricity using leftover biomass.

4. Wheat: Wheat is another potential feedstock for ethanol production. It contains a high amount of starch that can be enzymatically converted into fermentable sugars and then fermented into ethanol. However, wheat is primarily cultivated for food purposes and is a vital staple crop worldwide. Similar to potatoes and rice, using wheat for biofuel production may impact food supplies and prices, making it a less preferred choice compared to dedicated energy crops like sugarcane.

In summary, while all the mentioned options can be used to produce ethanol, sugarcane (Option 3) is the most commonly used and efficient feedstock for large-scale ethanol production. It offers high ethanol yields, efficient conversion processes, and potential co-generation of electricity, making it a viable choice for sustainable biofuel production.