Shifting cultivation is usually done where an area is

Shifting cultivation, also known as slash-and-burn agriculture, involves clearing a patch of land, cultivating it for a few years until soil fertility declines, and then abandoning it to move to a new area. This practice requires a large amount of land per person and is only sustainable where there is ample land available and a low population density.

Therefore, shifting cultivation is usually done where an area is sparsely populated.
The correct option is C.

Basin irrigation is a method of surface irrigation where water is impounded in level plots or basins. The options refer to water sources (tank, well) or the duration of irrigation (annual, perennial). While basin irrigation is a method, the question asks for a "type of irrigation system" it is another term for.

Perennial irrigation refers to an irrigation system that supplies water throughout the year, enabling continuous cultivation. Basin irrigation, when supplied by a reliable, continuous water source, can be a component of a perennial irrigation system. Among the given choices, "perennial" describes a system's operational characteristic that is often associated with the use of basin irrigation for year-round farming.
The correct option is D.

The Canadian Prairies are known for their vast, flat lands, which allow for large-scale, highly mechanized wheat farming. This type of farming uses relatively low labor input per unit of land to produce large quantities of a single crop.

This description fits extensive farming, which is characterized by large farm sizes, low capital and labor input per unit area, and relatively low yields per unit area.
The correct option is C.

Mixed farming is an agricultural system that involves both the growing of crops and the raising of livestock on the same farm.

Pastoral farming refers to raising livestock, and arable farming refers to growing crops. Therefore, mixed farming combines these two activities.
The correct option is A.

The description states that dried grass or leaves are placed on the soil around plants to prevent water loss, stop weeds, add nutrients when rotting, and reduce soil erosion.

This practice is known as mulching. Mulch is a layer of material applied to the surface of the soil.
The correct option is C.