The warning device of the brake system is activated when driving. What does this mean?

You selected both answer 1 ("Reserve pressure is no longer adequate") and answer 2 ("The brake system is probably defective"), which is correct. When the warning device for the brake system activates, it usually means there is a serious issue: either there is insufficient reserve pressure—meaning the system cannot generate enough force for safe braking—or there is a general defect in the brake system. Both cases are dangerous and require immediate attention, as proper brake function is crucial for safety. The third answer, "A brake cylinder is leaking," could be one possible cause, but the warning device indicates more generally that braking reliability is compromised, not necessarily this specific problem. Always treat any brake warning as urgent and have the system checked by a professional.