Under what conditions can a private pilot carry passengers for compensation?

A private pilot is generally prohibited from carrying passengers for compensation or hire, except under specific conditions. The correct answer indicates that carrying passengers can be permitted if it is incidental to a business and does not involve extra compensation. This means that if the flight is part of conducting legitimate business activities and no additional payment is made specifically for the flight itself, then the private pilot may carry passengers.

In contrast, the other options do not accurately reflect the regulations surrounding private pilot operations. The concept of "always permitted" contradicts the basic limitations placed on private pilots regarding compensation. Option C suggests that carrying passengers for compensation is only allowable under instrument flight rules, which is incorrect; the rules governing compensation do not change based on the type of flight rules. Lastly, option D implies that compliance with flight review regulations would allow for carrying passengers for pay, which is not a condition outlined in the regulations governing private pilot operations. Only certain conditions, like incidental business use without additional compensation, provide a clear exception to the general rule against compensation.