Which of the following property determines whether or not an element is visible?

The correct answer is A. display.

The display property determines the type of box generated by an element. It can be used to make an element visible, hidden, or to create a different type of box, such as a block, inline, or inline-block.

The visibility property determines whether or not an element is visible on the page. It can be used to make an element visible, hidden, or to make it appear as if it is hidden by setting it to hidden but still allowing it to affect the layout of the page.

The transparency property determines how transparent an element is. It can be used to make an element partially or completely transparent.

The disappear property is not a valid CSS property.