The correct option is D unpaired diencephalon
The brain of a frog is divided into three regions: forebrain, midbrain & hindbrain. The forebrain consists of cerebrum, olfactory lobes and unpaired diencephalon. Diencephalon is present at the posterior end of fore-brain. Optic lobes are a part of midbrain. Medulla oblongata is a part of hindbrain along with pons & cerebellum. Medulla oblongata passes through an opening present at the base of skull known as foramen magnum. The medulla oblongata continues posteriorly as spinal cord.