The correct answer is 'If subject wants to read and write to object, then subjects clearance and the objects classification must be equal.'. What makes the strong *property different is the ability of the subject to read or write at a different level. EDIT: According to Stewart, Chapple, Gibson (2015) pg 283 passim, in the Bell-LaPadula model, the star property allows write up and prohibits write down while the Biba model is the reverse, allowing write down and disallowing write up. The text does not explicitly mention strong star property but it is inferred when there is both Bell and Biba in place since the user is constrained on both ends only to their authorization level. Strong * property means you can only read and write at the same level. This is not the same as the * (star) property, which prevents a subject from writing down.