Which composition represents the lightest weight among the options?

To determine which composition represents the lightest weight, it's important to consider the molecular weights of the components involved. Dry air is primarily composed of nitrogen (approximately 78%) and oxygen (approximately 21%), which have a combined average molecular weight of about 29 g/mol. Water vapor (H₂O) has a molecular weight of approximately 18 g/mol.

In the problem, as the percentage of water vapor increases, the overall weight of the mixture decreases because water vapor is lighter than the components of dry air.

When examining the given options, the composition that mixes dry air and water vapor in the highest proportion of water vapor (while still having a reasonable amount of dry air) will have the lowest overall weight. The choice with 35 parts of dry air and 65 parts of water vapor achieves a high ratio of the lighter substance, making it lighter than mixtures with less water vapor. The balance here is ideal, as it contains enough dry air to be stable while maximizing water vapor.

Thus, this combination results in a lighter mixture compared to the others presented, making it the correct answer in terms of overall weight.