When DNA replication occurs, the DNA strand is being unwound, causing the DNA to be overwound tightly past the replication fork. For example, If you have ever wrapped two strings around each other and then tried to pull part the middle of the string while holding the ends tight, the rest of the string past the place you're pulling the string apart becomes very tight and strained.
Topoisomerase is an enzyme that binds to the DNA and helps release some of that strain by unwinding the DNA past the replication fork.
That means the correct box is the leftmost one. Hope this helps:)
