What the check is actually looking for
Three things break a small network more often than anything else: two end devices wired straight to each other with no switch, a device carrying more cables than it has ports, and a corner of the diagram with no path back to everything else. The check walks the graph outward from one device and reports whatever it cannot reach: the same question you end up asking when one floor of a building suddenly goes dark.
Learn the idea behind the lab
This lab is the practice; LANs, WANs & topologies covers the shapes networks come in, and How switches learn explains why the switch belongs in the middle.