What is the nut of an oak tree called ?

Acorn

The correct answer is Option (2)- Acorn

The nut of the oak tree, called the acorn, is made up of the cap, the tough outer shell, and the kernel inside. The tough outer shell and the cap of the acorn, called the cupule, protect the seed. The kernel is made of two fatty seed leaves, called cotyledons, which surround a small embryo in the pointed tip of the nut. Depending on the species, acorn nuts require anywhere from six months to two years to mature. Over a hundred animal species, including squirrels, rely on the acorns as an important food source. Some animals bury the acorns to store them for the winter. Not all of the buried acorns are dug up and some of them germinate and grow into mighty oak trees.