Which of the following is an example of a semi-postal stamp?

A semi-postal stamp is a type of postage stamp that is sold at a price higher than the standard postage rate. The additional amount collected is often used to fund specific causes or charitable projects. The Alzheimer's stamp fits this definition perfectly, as it was issued by the USPS to support Alzheimer's research and education, with a portion of the proceeds directly benefiting these initiatives.

In contrast, the Penny Black is a historical stamp known for being one of the first adhesive postage stamps, but it does not serve a charitable purpose. The Inverted Jenny is celebrated for its printing error and holds significant value among collectors; however, it is not a semi-postal stamp. The Christmas Stamp, on the other hand, typically serves the standard postage function and does not have an explicit surplus directed toward a charitable cause, making it distinct from semi-postal stamps.

Understanding this classification helps identify stamps that not only serve as postage but also contribute to broader social causes.