The correct option is A Unpaired diencephalon
The brain of a frog is divided into three regions- forebrain, midbrain and 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 and 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.
So, the correct answer is - unpaired diencephalon