Which of the following implements is not a tractor-coupled implement?

The sheller is not a tractor-coupled implement. A tractor-coupled implement is an equipment that is designed to be attached or hooked to a tractor to perform a specific task. For instance, a plough is an implement that is attached to a tractor to prepare the soil for planting by turning the soil over, while a harrow is used to smooth and level the soil after ploughing. A ridger, on the other hand, is used to form ridges on the soil for planting crops like potatoes. A sheller, however, is a machine that is used to remove the outer shell or husk of grains such as maize, beans, and peas. It is usually a standalone equipment that is powered by electricity or a diesel engine, and it does not require a tractor to function. Therefore, the sheller is not a tractor-coupled implement.