The correct answer is Ozone.
Trees does not release Ozone into the atmosphere.
Trees release oxygen through the process of photosynthesis whereby the oxygen is being breathed inside by human beings. Trees release water vapor through process of respiration.
Ozone is a gas which has pungent smell. Ozone is well known as allotrope of oxygen and it is not stable as diatomic allotrope.
Ozone is being formed by action of ultraviolet light which is from dioxygen.