Which of the following powers does the board have during an investigation?

The board has the authority to subpoena witnesses during an investigation, which is a crucial power for ensuring that the investigative process can proceed effectively. Subpoenaing witnesses allows the board to compel individuals to provide testimony and produce necessary documents that may be pertinent to the case. This power is essential for gathering all relevant information and evidence, ensuring that investigations are thorough and fair.

The options that involve imposing fines, filing criminal charges, or regulating therapy practices represent roles that may fall outside the board's direct investigative powers. The imposition of fines typically comes as a result of an established violation found after an investigation, rather than being an investigative power itself. Similarly, filing criminal charges is generally a function of law enforcement or prosecutorial bodies and not something done by a board during an investigation. Regulating therapy practices often pertains to the enactment of rules or standards, which may follow an investigation but is not an investigation-specific power. Thus, the ability to subpoena witnesses stands out as the key power that the board holds during their investigative activities.