The correct answer is option 2, Helium. A monotomic element refers to an element that exists as single atoms, rather than molecules. Copper (option 1), iodine (option 3), and barium (option 4) are not monotomic elements because they form molecules consisting of more than one atom.

Copper atoms combine to form copper molecules, Iodine forms diatomic molecules (I2), and barium forms metallic bonds in a lattice structure. On the other hand, helium (option 2) is a noble gas and exists as individual helium atoms. Noble gases are known as "inert gases" because they do not readily react with other elements due to their stable electron configurations. Helium is a colorless, odorless, and non-toxic gas that is lighter than air. It is commonly used in balloons and as a cooling agent in various applications due to its low boiling point and high thermal conductivity.

Therefore, among the given options, Helium is the only monotomic element.