What attribute can be used to specify the height of an iframe?

The height attribute is specifically designed for iframes to define their vertical size. When you use this attribute, you can set the height of the iframe in either pixels or percentage values, allowing you to control how much vertical space it occupies on the webpage. This is important for layout and visual presentation, ensuring that the content within the iframe is displayed properly without unnecessary scrolling or distortion.

For context, while width specifies the horizontal size of the iframe, the size and scale options do not apply directly to the iframe element and do not determine its dimensions. Thus, height is the only correct attribute from the choices given that directly affects the vertical dimension of an iframe.