The correct option is A Foramen magnum
The central nervous system of frog is composed of the brain and the nerve cord. The brain is enclosed in a bony structure called the cranium or brain box. The brain is divided into forebrain, midbrain and hindbrain. The hindbrain consists of cerebellum and medulla oblongata. The medulla passes through the foramen magnum and continues into the spinal cord and the spinal cord is enclosed in the vertebral column.
So, the correct answer is option B.