You are missing the point of this part of the test. It is a grammar test. You will need to use a book that helps you with basic grammar.
A run-on sentence is a sentence in which two or more independent clauses (that is, complete sentences) are joined without appropriate punctuation or conjunction.
Marie aime les croissants son chat préfère du lait. >>> What is missing here? A conjunction or a period.
For reference, it is a form of parataxe or "phrases coupées " that is NOT accepted in English. Camus might have liked it.