Meaning: Greywater is the wastewater from sinks, showers, and washing machines that can be reused for non-potable purposes.

Wrong options:

• reusable: While 'reusable' is an adjective that describes something that can be used again, it does not specify the type of systems being referred to in the context of water conservation.
• aquatic: 'Aquatic' is an adjective relating to water but does not fit as it does not describe a system for collecting rainwater.
• filtration: 'Filtration' is a noun that refers to the process of removing impurities from water, but it does not specify the type of system being discussed.

Correct answer: efficient

Explanation: The word 'efficient' is an adjective that describes products designed to use less water while maintaining performance. This fits grammatically as it modifies 'products' and aligns with the context of minimizing waste in households.

Meaning: Efficient means using resources in a way that maximizes productivity with minimal waste.

Wrong options:

• reusable: 'Reusable' is an adjective that describes items that can be used multiple times, but it does not specifically convey the idea of minimizing water usage effectively.
• soaring: 'Soaring' is an adjective that describes something that is rapidly increasing, which does not fit the context of describing household products.
• aquatic: 'Aquatic' is an adjective related to water but does not describe the efficiency of products in conserving water.

Correct answer: soaring

Explanation: The term 'soaring' is an adjective that describes the rapidly increasing demands of urban populations for water. This fits grammatically as it modifies 'demands' and is contextually relevant to the discussion of water conservation challenges.

Meaning: Soaring means rapidly increasing or rising to a high level.

Wrong options:

• efficient: 'Efficient' is an adjective that describes the effectiveness of products, which does not fit as it does not describe the demands of populations.
• essential: 'Essential' is an adjective that means absolutely necessary, but it does not convey the idea of increasing demands.
• reusable: 'Reusable' is an adjective that describes items that can be used again, which does not relate to the concept of demand.

Correct answer: essential

Explanation: The word 'essential' is an adjective that describes resources that are absolutely necessary for survival and well-being. This fits grammatically as it modifies 'resources' and aligns with the context of discussing the importance of conserving water for future generations.

Meaning: Essential means absolutely necessary or extremely important.

Wrong options:

• soaring: 'Soaring' describes something that is rapidly increasing, which does not fit as it does not describe the nature of resources.
• efficient: 'Efficient' is an adjective that refers to effectiveness, which does not convey the necessity of resources.
• greywater: 'Greywater' is a noun referring to a specific type of wastewater, which does not fit the context of describing the importance of resources.