What is type-safety?

I had a brainbench exam recently, got high mark, but there were a couple of questions which were hard for me. Maybe it's because english is not my native language... One of the questions is: Which one of the following describes type-safety? - A programming construct used to ensure the security of reference and value types in the CLR - The protection from memory leakage as a result of disallowing unmanaged access - The CLR-specific feature providing assurances that types may not access memory outside their own AppDomain - A mechanism created to protect assemblies and their types by the use of strong-named keys - The concept dealing with assurances that allocated objects are always accessed in compatible ways I think it's 1 or 5, but they sound weird to me anyway :( What do you think?