The interviewees were slowly becoming impatient as they were left to _____ in the waiting room for hours. 

cool their heels

The correct idiom to fill in the blank is cool their heels.

This idiom means to wait patiently, especially in an uncomfortable or boring situation. It is the most appropriate option in this context because it conveys the idea that the interviewees were becoming restless and frustrated as they were left to wait for hours.

The other options are incorrect.

• Draw the long bow means to exaggerate or make something seem more important than it really is. It does not make sense to say that the interviewees were drawing the long bow by waiting in the waiting room for hours.
• Hold water means to be true or accurate. It does not make sense to say that the interviewees were holding water by waiting in the waiting room for hours.
• Flog a dead horse means to continue to do something that is pointless or unsuccessful. It does not make sense to say that the interviewees were flogging a dead horse by waiting in the waiting room for hours.