The question is from the following problem:
If $f$ is the function whose graph is indicated in the figure above, then the least upper bound (supremum) of $\big\{\sum_{k=1}^n|f(x_k)-f(x_{k-1})|:0=x_0
$A. 2\quad B. 7\quad C. 12\quad D. 16\quad E. 21$
I don't know what the set above means. And I am curious about the background of the set in real analysis.