Which of the API object IDs listed below is in an incorrect format?

The correct choice identifies an API object ID that does not adhere to the typical formatting standards used by Stripe. In Stripe, object IDs generally consist of a combination of alphanumeric characters and often incorporate specific patterns.

The ID format for many Stripe object types typically follows a pattern that includes a combination of lowercase letters and hyphens. For example, IDs like "abc12345" and "xyz-6789" fit within expected conventions, with lowercase letters and numbers, and in the case of the latter, a hyphen that conforms to the common token format.

In contrast, "ABCD1234_req" includes an underscore and uppercase letters, which deviates from the standard format seen in Stripe's ID conventions. The presence of uppercase letters and the underscore is indicative that this ID does not conform to the required structure for API object IDs, resulting in the identification of this option as incorrect.

Therefore, the ID format of "ABCD1234_req" does not match the expected formats for Stripe API object IDs, making it the correct choice for this question.