Which phrase describes the essence of 'Penny Black'?

The essence of 'Penny Black' is most accurately captured by the phrase that identifies it as the first adhesive postage stamp. Issued in the United Kingdom on May 1, 1840, the Penny Black revolutionized the way postage was handled, making it more accessible to the general public by introducing a pre-paid system for sending letters. This marked a significant shift in postal services, as it removed the burden of payment from the recipient to the sender and facilitated more efficient mail delivery.

While the other choices touch on aspects of postal or monetary history, they do not accurately reflect the groundbreaking nature of the Penny Black stamp. For instance, it was not a decorative coin nor was it the first stamp used by the UK government; it was specifically the first of its kind to be used as an adhesive postage stamp—an innovation that has had a lasting impact on global postal systems. This foundational role in the development of mail services underscores why identifying it as the first adhesive postage stamp is the most precise description of its essence.