Correct Answer: impervious

Solution : The fourth option is the correct choice.

Impervious means not allowing fluid to pass through; unable to be affected by. In the context of proportioning sand for a mixture, impervious suggests creating a mixture that is resistant to the penetration or influence of external factors, making it the most appropriate word.

The meanings of the other options are as follows:

• Envious means feeling discontent or resentment towards someone else's possessions, qualities, or luck.

• Obvious means easily perceived or understood; clear.

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• Devious means showing a skillful use of underhanded tactics to achieve goals.