Final answer:
Rhiannon is most likely using Canva, a web-based graphic design tool ideal for creating business advertising materials, which is user-friendly and accessible online unlike Photoshop, Illustrator, or GIMP.
Step-by-step explanation:
Rhiannon is likely using Canva for creating a poster to advertise her new business using an online graphic design tool. Unlike Photoshop and Illustrator, which are professional-grade software requiring installation on a computer, Canva is known for its user-friendly, web-based interface that makes it easy for beginners and non-designers to create graphics for various purposes including posters for business advertising. GIMP is also a free and open-source software but it requires downloading and is more complex like Photoshop and Illustrator.