......................, also known as the breath of life is essential for the chemical processes that sustain life.
Oxygen is essential for various life sustaining chemical reactions. Oxygen is used in respiration, a process by which energy is formed in the form of ATP in the body needed for physical work and survival. Oxygen is required by all tissues in body and lack of oxygen in cells and tissues can lead to exhaustion and eve death of tissue. Hence, oxygen is also known as breath of life.
So, the correct answer is 'oxygen'.