Miller's classic experiment demonstrated that a discharge of sparks through a mixture of gases could result in the formation of a large variety of organic compounds. miller did not use as one of the gases in his experiment.

The correct answer will be-gas absent in primordial atmosphere (oxygen).

Explanation:

The Miller experiment was performed in 1953 to show that organic molecules can be formed from the inorganic molecules in the conditions resembling the primordial atmosphere.

Same conditions were created to the unstable atmosphere like a mixture of gases which included- methane, sulfur dioxide and hydrogen but lacked oxygen as it was absent in the primordial atmosphere.

Thus, gas absent in the primordial atmosphere (oxygen) is the correct answer.

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