Answer :
The condition at which the reaction to become darker brown is:
d. High Temperature
On adding Heat:
Usually, when you add energy to a bunch of atoms they move faster and get hotter. Similarly, if you remove energy from a bunch of atoms, they usually move less and get cooler. Because adding heat energy usually results in a temperature rise, people often confuse heat and temperature.
When heat is added to a brown 2 NO₂ gas to yield a colorless N₂O₄ gas
Thus, correct option is d.
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