Generally there are two kinds of bleaching agents, notably chlorine (and many chlorine compounds such as sodium hypochlorite) and peroxygen bleaching agents (such as hydrogen peroxide). This isn't to say there aren't others, because there are, such as ammonia. (Note, mixing traditional bleaches and ammonia is very dangerous)
Answer: a. nitrogen
Explanation:
Nitrogen comprises of five valence electrons in its outermost shell which enables it to form triple, double and single bonds with other atoms. It can share three electrons with other nitrogen atom and form a covalent bond. Moreover, nitrogen can form multiple bonds with itself and with other elements due to its small size and high electronegativity. Nitrogen, carbon, fluorine, and oxygen are the elements with which nitrogen can form bonding.