Which of the following API object IDs represents an incorrect format?

The API object IDs must adhere to specific formats that typically include alphanumeric characters and may have certain delimiters. In this instance, the correct format should not begin with a purely alphabetical sequence followed abruptly by numerical characters without any delineation.

In the first option, "ABCD1234_req," although it contains letters followed by numbers, the suffix "_req" introduces an underscore which is commonly not used or expected in many API object identifiers. This format deviates from the more standardized representation that usually consists of a clear joining of letters and numbers without additional characters.

On the other hand, the other options maintain formats that are generally acceptable within API conventions. "XYZ9876-confirm" and "PQR5678-accepted" both use a combination of letters and numbers followed by a hyphen and another word, which are recognized as valid identifiers that can describe various states or types of requests. The format of "LMN91011" is also clear, showcasing a straightforward numeric sequence following the characters.

Thus, while other options display valid combinations of alphanumeric characters and acceptable suffix formats, the first one does not conform to typical expectations for API object identifiers due to the use of the underscore.