The correct answer is option 4: Modified fore limbs.

Birds have evolved from a group of dinosaurs known as theropods, which had forelimbs adapted for grasping or holding objects. Over time, these forelimbs have been modified into wings to enable birds to fly. The bones in a bird`s forelimb, such as the humerus, radius, and ulna, have undergone changes to support flight. Feathers, which are a defining characteristic of birds, are attached to the modified forelimbs and contribute to the bird`s ability to fly. These modifications in the forelimbs of birds have allowed them to become highly skilled and efficient flyers.

Therefore, it can be concluded that the wings of birds are modified fore limbs.