Which type of documents must be accessible during an investigation by the board?

The correct answer is treatment notes and records, as these documents are essential for the board to evaluate the standards of care provided by a chiropractor during an investigation. Treatment notes and records offer insights into the diagnosis, treatment plans, and patient interactions, which are critical for assessing whether appropriate care was delivered and whether the chiropractor adhered to professional standards and regulations.

In contrast, while employee handbooks may provide some relevant operational information, they do not directly pertain to the individual patient's care or the chiropractic services rendered. Personal diaries and internal financial reports are unrelated to the professional conduct of chiropractic treatment and are generally not necessary for an investigation focused on patient care standards. Therefore, focusing on treatment notes and records reflects the board’s primary concern with patient safety and care quality.