Final answer:
The most likely cause for a change in a graph regarding congressional districts is Congressional redistricting, as states redraw district boundaries to favor certain political parties, a process known as gerrymandering.
Step-by-step explanation:
The change seen in the graph would best be accountof Representatives, this is based on population as determined by the National Census every ten years, according to Section 2 Clause 3 of the U.S. Constitution. Given that states cannot gain more districts than they have been apportioned, and considering that states can gain but never lose districts, the most reasonable explanation for a significant change in a graph representing congressional districts would be that redistricting had occurred. This process, ofteed for by Congressional redistricting having taken place. When referring to the apportionment of seats in the House n called gerrymandering, involves state legislatures drawing district lines in a way that favors a certain political party. This can significantly alter which party wins a given district’s House seat without changing the absolute number of seats a state has, which is why Congressional redistricting is a more likely explanation than a change in laws regarding apportionment or a change in the Legislative branch's power.