Answer:
A. a box sketched on a wireframe to show where content or images will go.
Explanation:
A placeholder is way to represent the space used by a text or an image within the global layout when you don't have yet the content that goes there. It "holds the place" of a future element to be added later.
For images, you can just place a grey image showing there will be something there, in those dimensions, in the future. Avoid placing an actual image not to be confused later and think the layout is complete, when you are missing an image.
For text, you can either use a blank space or filler text like "Lorem ispsum...", which would have the benefit of showing the font to be used.
In HTML, a placeholder is also used with input text fields to present an example of the data we're looking for.
