What property is used to specify the number of columns in a CSS grid layout?

The grid-template-columns property is used to define the number of columns in a CSS grid layout. By specifying this property, you can set the width of each column, allowing for a flexible and responsive design structure. For instance, you can indicate a specific number of columns by using values like "repeat(3, 1fr)" to create three equal-width columns or "100px 200px" to define columns of fixed widths. This property is crucial for establishing the overall grid structure, which organizes content in a two-dimensional grid format.

The other choices serve different purposes within the context of a CSS grid layout. For example, grid-template-rows is designated for defining the height of rows, while grid-area is used to assign elements to specific areas within the grid. Grid-auto-flow helps to control how items are placed in the grid when there's no explicit position defined for them. Hence, while each property is important in its own right, grid-template-columns specifically addresses the vertical organization of columns in a grid layout.