Strengthen questions just ask us to improve the argument in some way. They can phrase it in many ways but at their core they are just asking for what would make the argument or conclusion stronger.
Sufficient assumption question ask us what would make the argument right. They have several ways of doing so, often asking us to justify the argument, but at its core they ask us what makes the conclusion correct.
Necessary assumption questions ask us what the author must believe. They can do so in several ways, most common are to ask what the argument assumes or what the argument requires.
