A disadvantage of a projected teaching aid is:

The correct answer is that a disadvantage of a projected teaching aid is the necessity of a clean environment. This is particularly relevant because projectors require a clear surface or screen to display images effectively. If the screen is dirty or marked, it can hinder visibility and distract the audience from the content being presented. A clean environment is essential to ensure that the projected images are clear and that the audience can engage with the material without undue distraction.

Other factors, while potentially relevant, do not particularly stand out as inherent disadvantages specific to projected teaching aids. For instance, the need for specific lighting conditions, like a darker room for clearer projection, isn't typically considered a disadvantage in the same way; rather, it is an operational consideration. Additionally, while projected teaching aids can be less effective in very bright environments or for small audiences, they are not strictly limited to those scenarios and can actually be effective in various contexts with appropriate adjustments.