On the PPF, the unemployment rate is equal to the natural rate of unemployment which consists of frictional unemployment and structural employment but excludes cyclical unemployment. 

The move from PPF1 to PPF2 represents an increase in the productive potential(possibilities) of the economy. This can be effected by an increase in the productive resources in the economy, which would be represented by an increase in the labour force. 

The diagram shows an increase in the potential productive capacity of the consumer good production without a corresponding increase in the productive capacity of capital good production. The best answer then is that whatever change happened affected only the capacity to produce consumer goods. Labour force increases would increase the capacity to produce both goods, and so would an economic boom.

A tariff, also known as a "border tax", is a tax placed only on specific goods being imported into a country. A tariff can be used to restrict foreign imports, harm foreign producers in some way, protect domestic businesses, or raise additional revenue. What a tariff never does is open markets and promote absolute free trade.

The short-run Phillips curve indicates an inverse relationship between inflation and unemployment. According to the short-run Phillips curve, as unemployment goes up, inflation goes down, and as inflation goes up, unemployment goes down.

The correct answer is therefore "A tradeoff between unemployment and inflation." Recall that a tradeoff refers to an inverse relationship.

The other answer choices are all distortions of the predictions of the short-run Phillips curve. 

The Phillips Curve is meant to express the short-run tradeoff between inflation and unemployment.

An increase in worker productivity will result in the supply curve shifting, because it results in the economy having more potential output.

A rightward shift of the demand curve (i.e. an increase of the demand curve) causes price and quantity to increase.

Since the aggregate demand/aggregate supply (AD/AS) model represents price as price level and quantity as output, a rightward shift of the aggregate demand curve results in an increase in the price level and an increase in output.

If you selected "A decrease in output and a decrease in the price level" you may have found the effects after a leftward, rather than rightward, shift of the aggregate demand curve.

Answer choices "An increase in output and a decrease in price level" and "A decrease in output and an increase in price level" are incorrect because shifts in demand cause both price and output to rise simultaneously or fall simultaneously, but never cause price to rise and quantity to fall or price to fall and quantity to rise.

A progressive tax system is so-called because, as a person or corporation makes more money, the overall tax burden as a percent of income gets progressively higher. A progressive tax seeks to produce more government revenue from those individuals who are perceived as being more able to bear the burden. Progressive taxation is usually popular politically, making it such a widespread form of taxation.

Tax law includes small exemptions and refunds, which are known as tax credits. Often, these tax credits are provided for groups of people like homebuyers, parents, and students. Tax credits are also usually seen as a way to lighten specific tax burdens without rewriting or transforming tax policy.