Which type of call sign has a single letter in both the prefix and suffix?

The question asks about a specific feature of call signs, particularly those that contain a single letter in both the prefix and suffix. A special event call sign is used for a temporary operation during an event, and it typically includes unique and often memorable combinations of letters and numbers. These call signs can sometimes feature a single letter at both ends, which helps to distinguish them from regular call signs and draws attention to the special nature of the operation.

Vanity call signs are personalized, but they don't necessarily follow the pattern of having a single letter in both the prefix and suffix. Sequential call signs are assigned in order and maintain a standard format that does not include this feature either. In-memoriam call signs are specific tributes but also don't conform to the single-letter requirement. Therefore, the answer indicating that a special event call sign can achieve this unique configuration makes it the most appropriate choice.