I do know which chicken who has those droppings (her name is Marge) and also, if they are tapeworms, can they be transmitted to humans?
No they cannot be transmitted to humans. They are poultry specific.
It is Spring in your part of the world, right? That is generally when tapeworm populations peak in free-range chickens. Chicken acquire tapeworms from eating insects (which are numerous in the spring and summer) which carry immature tapeworms. The tapeworms then reach their adult stage inside the chicken, where they then grow and shed the "segments" you see in the poop. Insects eat the segments, chickens eat the insects, and the cycle continues. This makes treatment this time of year really tough, because although you can kill the adult tapeworms living in your chicken, the chickens will continuously re-infect themselves by eating new insects. It takes about 2 or 3 weeks for an adult tapeworm to grow after your chicken eats an insect.
OK, so what to do. First, make sure Marge really has tapeworms. Although some tapeworms are microscopic, in the most common types, the segments are visible to the naked eye. Most segments are shed in the middle of the day, so if you have a way to temporarily confine her, you can check her droppings over the course of day. Tapes shed segments every day, so if she has them, you all see them.
If she has them, the praziquantel will kill them, and
@dawg53 is an expert when it comes to treating them. Be forewarned, they will probably come back in a month after the treatment has ended unless you can figure out a way to control the insect population or keep your chickens away from the insects. Them most common carriers are beetles and ants.
Sometimes tapeworms go away on their own, as the seasons change and chickens build resistance. That was my experience at least and I wrote about it here
https://www.backyardchickens.com/threads/the-case-of-the-disappearing-tapeworms.1337076/. The wait and see approach may not be right for you, however. A lot depends on how healthy your chickens look and your own personal risk tolerance.
Ugh, I really hope it's not tapeworms!