Explanation:

- The phrase "He had barely spoken the words when he realised his mistake" is used to indicate that the realization happened almost immediately after speaking.

- Option 1: "barely speak the words that" is incorrect because it doesn't fit the past perfect tense and makes the sentence awkward.

- Option 2: No substitution indicates the original sentence is correct and grammatically appropriate.

- Option 3: "barely spoken the words than" is incorrect because "than" is not correctly used in this context; "when" should be used.

- Option 4: "barely spoke the words when" suggests a simple past tense, but the original meaning with past perfect is stronger.

- The original sentence accurately expresses a quick sequence of events and is the best choice.