The US periodically alters/adjusts the number of representatives in the House, called "reapportionment". What do they base this periodic "adjustment" on?

It is based on the population of the area

Explanation:

In the United States Congress, the process of redistribution of the seats for the U.S. house of Representatives which is based on the changes on the population of the place. Some communities have increased in population or grown ans such communities or places will gain representations in the Congress while communities that have shrunk will loss a seat.

This is done by the Federal government counting the number of people living in the States in every 10 years.

even though you have attached neither the chart nor the generalizations, i have located them online. the right answer is d, since they are asking you about a generalization. you can only infer from the chart that between 2008 and 2009 the number of employed workers decreased significantly (from over 115 millions to 107 millions approximately), which signals some type of economic crisis.