Short-run Phillips curve shows:

A vertical line would imply that there is no trade-off between inflation and unemployment, which contradicts the fundamental premise of the short-run Phillips curve. In the long run, this may hold true due to natural rates of unemployment, but the short-run curve is specifically characterized by its downward slope, reflecting the temporary trade-off.

An upward slope would suggest that as inflation increases, unemployment also increases, which is contrary to the observed relationship depicted by the short-run Phillips curve. This choice misrepresents the economic dynamics at play, as historically, higher inflation rates have been associated with lower unemployment in the short term.

A horizontal line would indicate that changes in inflation do not affect unemployment, which is not representative of the short-run dynamics captured by the Phillips curve. This choice overlooks the critical interaction between these two variables that the curve illustrates, particularly in the short run.