a. The inflation rate is constant.
Explanation:
In the earlys of the xx century, Phillips developed a curve which linked a negative relation between unemployement rate and inflation. Over the time, the predictions of the curve failed, and that is how a new theoretical approach came across. The NAIRU (Non-Accelerating Inflation Rate of Unemployment) defines a unemployement rate where the inflation rate is constant, rejecting the traditional assumptions of the Phillips's curve.
Please take a look to the attached graph to understand the concept.
![The nairu is the rate of unemployment at which: a. the inflation rate is constant. b. unemployment](/tpl/images/0224/0056/7f012.jpg)