The correct answer is B) braided layer. The braided layer is the determining factor for the strength of a flexible rubber hydraulic hose. This braided layer, typically made of high-strength steel wire or synthetic fibers, provides the necessary tensile strength to withstand the high pressures and forces exerted on the hose during operation. The other components, such as the Teflon sleeve, synthetic rubber inner tube, and external cover, serve other purposes like providing chemical resistance, flexibility, and protection, but do not primarily determine the overall strength of the hose.