Hint: We should keep in mind the definition of what is a reflex arc. It is the neuronal network that controls a reflex. Most sensory nerves in vertebrates synapse in the spinal cord rather than passing straight through the cortex. This enables quicker reflex movements by triggering spinal motor neurons without having to wait for impulses to travel through the brain.
Complete solution: A reflex activity always requires a reflex arc, which is a very basic nervous pathway. Receptors are excited at the onset of a reflex arc. They then transmit messages to the spinal cord from a sensory neuron, which is then passed on to a motor neuron. Involuntary and almost instantaneous reaction in response to a stimulus is known as reflex motion. Invertebrates' sensory neurons synapse in the spinal cord rather than passing straight through the cortex. The spinal cord is the primary control centre for reflex behaviour. The spinal cord connects the brain and the spinal nerves.
So we can say that, reflex arc is controlled by the spinal cord.
Note: Remember that reflex action is regulated by a neuronal pathway. The majority of sensory nerves in higher species do not flow straight through the cortex, but rather synapse in the spinal cord. This property helps reflex movements to happen immediately by triggering spinal motor neurons without having to wait for impulses to be routed into the brain, but the brain will also get sensory feedback during the reflex action.