Which type of layer can be used to apply effects to multiple layers beneath it?

An adjustment layer is a special type of layer in Adobe After Effects that allows you to apply effects to all layers located beneath it in the layer stack. When you create an adjustment layer and apply an effect to it, that effect applies to every layer underneath, effectively allowing for a wide range of creative possibilities without having to apply the same effect individually to each layer. This is particularly useful for global adjustments, such as color correction, blurring, or adding a specific visual style to a composition.

While solid layers, text layers, and video layers have their distinct functions—such as serving as backgrounds, containing text elements, or holding video footage—none of them provide the ability to universally modify the appearance of multiple other layers in the same way that an adjustment layer does. Thus, the adjustment layer is specifically designed for this purpose, making it the correct choice for the question.