How did Dahl confirm the exact dates of his training?

The correct answer highlights that Dahl confirmed the exact dates of his training by consulting his logbook. Logbooks are formal records kept to document detailed information about activities and experiences, which would provide precise dates and notes related to his training. This method ensures accuracy and reliability, as logbooks are typically maintained in a systematic manner, making them an excellent source for recalling specific times and events.

In contrast, while personal letters, significant memories tied to events, and diaries can provide context and general timelines, they may not be as reliable or specific as a logbook when it comes to confirming exact dates. Each of those options can be influenced by personal memory bias or lack of detail, which is why the use of a logbook is the most accurate method for this purpose.