Just enquiring as to your reasoning for Q19 (believing B is correct)
The NAIRU is the level of unemployment when cyclical unemployment is zero. It is comprised of structural and frictional factors.
How would a down turn in the business cycle have any bearing on the NAIRU which is by definition independent of cyclical factors.
The correct answer for what we know is definitely C.
However, if he says that in university you learn it's B, then the only possible reasoning I could come up with is that an economic downturn leads to not only cyclical unemployment, but ALSO structural unemployment.
This is because an economic downturn will force resource allocation to become more efficient, and the least inefficient industries will be forced to cull their labour force, which is essentially micro reform and thus structural unemployment.
As a result of this, there are now less jobs on offer than before the downturn in the economy.
Hence, the natural rate of unemployment (or the NAIRU) has increased, because FULL employment now occurs at a lower percentage.
At least, that's how I could possibly see it working.
Nevertheless, answer for 19 is C.