I prefer to keep pre-assessments short and simple and have found supported multiple choice questions a great way to tune-in to the student’s current level of understanding.
For this question the students not only have to choose which answer is correct, they have to justify their choice, along with proving why the other options are incorrect. (printable version)
I posted the students’ work in an accessible place so, throughout the unit, they could add their ideas as we went. This was a great tool for on-going assessment of their conceptual understanding and also their use of mathematical language.
Doing Maths 