What is the effect of the 'opacity' property in CSS?

The 'opacity' property in CSS is specifically designed to control the transparency of an element. When you apply the opacity property, you set a value between 0 and 1, where 0 means completely transparent and 1 means fully opaque. This allows for a nuanced display where elements can be partially see-through, blending them visually with whatever is underneath them on the page.

For instance, an opacity value of 0.5 will render the element semi-transparent, making it easier to see the content or background behind it. This capability is particularly useful for various design effects, such as layering images, creating overlays, or enhancing user interface elements by allowing backgrounds to show through.

The other options do not align with the specific function of the opacity property. While changing visibility might suggest an element is hidden or shown, and adjusting brightness relates more to filter effects, these are not what the opacity property does. Similarly, altering the position refers to properties like 'top', 'left', 'margin', or 'padding', which control layout rather than transparency.