Under which condition can a straight-in approach be flown according to ATC regulations?

A straight-in approach can be flown under various conditions as delineated by ATC regulations, making the selection of "any of the above circumstances" the correct choice.

When issued an ATC clearance for a straight-in approach, the pilot is instructed to conduct the approach according to the published procedures, which indicates that ATC has determined it is safe and appropriate to do so.

Moreover, being established in an assigned holding pattern demonstrates that the aircraft is within the parameters defined by ATC and may be cleared for a straight-in approach once the proper sequencing and conditions are met.

Also, if cleared by ATC radar vectors to a final approach position, it signifies that ATC is actively managing the aircraft's path, ensuring that it is adequately positioned for a successful landing. Each of these conditions ensures that ATC has given the necessary instructions or has established situational awareness that allows for a straight-in approach. Therefore, any of the scenarios provided are valid justifications for conducting a straight-in approach under ATC regulations.