Sugarcane, Potato, and Tapioca are energy crops.

Energy crops are plants that are specifically cultivated for the purpose of producing biofuels or generating biomass energy. These crops have the ability to convert solar energy into chemical energy through photosynthesis, and they are rich in carbohydrates and other organic compounds that can be used as a source of energy.

Sugarcane:
Sugarcane is a widely grown energy crop. It is primarily cultivated for the production of sugar, but it is also an excellent source of biofuel. Sugarcane is used to produce ethanol, a renewable fuel that can be blended with gasoline or used as a standalone fuel. Ethanol produced from sugarcane is a cleaner and more sustainable alternative to fossil fuels.

Potato:
Potato is another energy crop that can be used to produce biofuels. Although potatoes are primarily consumed as a food crop, they can also be converted into ethanol. Potatoes contain starch, which can be broken down into sugars and fermented to produce ethanol. Potato-based ethanol can be used as a fuel additive or as a substitute for gasoline.

Tapioca:
Tapioca, also known as cassava, is a tropical crop that is widely cultivated for its starchy roots. Like potatoes, tapioca can be processed to extract starch, which can then be converted into ethanol. Tapioca-based ethanol is a valuable biofuel that can be used to reduce greenhouse gas emissions and dependence on fossil fuels.

Millets, Banana, and Tomato:
Millets, bananas, and tomatoes are not typically considered energy crops. Millets are primarily grown as food crops, while bananas and tomatoes are consumed as fruits and vegetables. Although these crops have nutritional value, they are not commonly used for biofuel production or energy generation.

In conclusion, the correct answer is option 'C' (Sugarcane, Potato, and Tapioca) as these crops are commonly cultivated for their energy-producing potential, specifically for the production of biofuels like ethanol.