The medulla oblongata sits at the transition zone between the brain and the spinal cord.
Medulla oblongata receives and integrates signals from spinal cord and sends resulting signals to the cerebellum and thalamus.
It contains centres that regulate heart rate, blood pressure, breathing, swallowing, salivation, sneezing, vomiting and coughing and other involuntary movements.
Therefore, medulla oblongata originates from ectoderm. Hence, the correct answer is option a.