In order to calculate Shelley's average velocity, first let's find the distance between the start and finish point, using the Pythagorean theorem:
![\begin{gathered} x^2=250^2+575^2 \\ x^2=62500+330.625 \\ x^2=393125 \\ x=627 \end{gathered}](https://img.qammunity.org/2023/formulas/physics/college/chqhx3qr1cm0sny9rsg6nwbkj19ex7eeiw.png)
Then, dividing this distance by the total amount of time spent, we have:
![\begin{gathered} \text{velocity}=(x)/(45+55) \\ \text{velocity}=(627)/(100) \\ \text{velocity}=6.3\text{ m/s} \end{gathered}](https://img.qammunity.org/2023/formulas/physics/college/vk8qxj46qejufkdwygdkn9qypwdx17rj1i.png)
Therefore the correct option is C.