By a surface of a liquid we mean
(a) a geometrical plane like x = 0
(b) all molecules exposed to the atmosphere
(c) a layer of thickness of the order of 10-8m
(d) a layer of thickness of the order of 10-4m.

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1 Answers

(c) a layer of thickness of the order of 10-8

Explanation: 

The surface tension in a liquid is effective to a 10 to 15 molecular depth from the open surface. Hence in the context of surface tension, the surface of a liquid is meant by this much thick layer which comes to the order of 10⁻⁸ m.

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