Select the most appropriate option to fill in the blank.

Very special care should be taken to proportion the sand so as to make a perfectly _______ mixture.

The correct answer is 'Option 2' i.e. 'Impervious'.

Key Points

• The correct answer is: 'Impervious'
• In the context of this sentence, the term "impervious" is the most fitting.
• "Impervious" means "not allowing fluid to pass through." When preparing a mixture like concrete or mortar. 
• For example, it's important to ensure the mixture is "impervious" so it can effectively resist water penetration.

Therefore, the correct sentence is: 'Very special care should be taken to proportion the sand so as to make a perfectly impervious mixture.'

Additional Information

• Now, let's review the other options:
  • Devious - This adjective usually describes something or someone that is dishonest or untrustworthy, and it doesn't make sense in the context of a mixture.
  • Obvious - This adjective means "easily perceived or understood," and doesn't make sense in this context. You wouldn't want a mixture to be 'obvious.'
  • Envious - This term is used to describe a feeling of desire for someone else's possessions or advantages, and it doesn't apply to a mixture.