The correct answer is C) stiffeners. Reinforcing frames attached to a bulkhead on a vessel are called stiffeners. Stiffeners are structural elements that provide additional support and rigidity to a bulkhead or other surface, preventing it from deforming or buckling under stress. This is a key concept in the construction and design of vessels, as stiffeners play a crucial role in the overall structural integrity of the ship. The other options are incorrect because: A) Brackets are used to support and attach other components, but are not the term specifically used for reinforcing frames on a bulkhead. B) Intercostals refer to transverse framing members between longitudinal members, not reinforcing frames on a bulkhead. D) Side longitudinals are lengthwise structural members, not the reinforcing frames attached to a bulkhead.