What does the flat part of the graph represent

Which of the functions below could have created this graph?

Delbert keeps track of total of the total number of points he earns on homework assignments, each of which is worth 60 points. at the end of the semester he has 810 points. write an equation for delbert’s average homework score a in terms of the number of assignments n.

What is the effect on the graph of the function f(x) = x2 when f(x) is changed to f(x) − 4?