The correct answer is option 1 - upper incisor. The tusk of an elephant is an enormously enlarged upper incisor. Incisors are the sharp front teeth used for cutting and biting. In elephants, the upper incisor on each side of the mouth grows continuously throughout their lives, protruding outwards to form tusks. These tusks are made of ivory, a hard, dense material. Elephants use their tusks for various purposes, including digging, lifting objects, defending themselves, and foraging for food. It is important to note that only male elephants usually develop tusks, while female elephants, known as cows, may have very small or no tusks at all.