Meaning: 'Sugar' refers to a sweet, soluble carbohydrate that is produced by plants during photosynthesis.

Wrong options:

• nutrient: 'Nutrient' is a broader term that refers to substances that provide nourishment, but it does not specifically describe glucose as a type, making it less precise in this context.
• scarcity: 'Scarcity' refers to a lack of something, which does not fit as it does not describe what glucose is.
• temperature: 'Temperature' is a noun that refers to the degree of heat, which is unrelated to the classification of glucose.
• oxygen: 'Oxygen' is a gas produced during photosynthesis, but it does not describe glucose and is therefore irrelevant here.
• enzyme: 'Enzyme' refers to a biological catalyst, which does not relate to the type of substance glucose is.

Correct answer: oxygen

Explanation: The term 'oxygen' is a noun that fits grammatically as it serves as the subject of the clause 'produces __ as a by-product.' Contextually, it is accurate because oxygen is indeed a by-product of photosynthesis, essential for life.

Meaning: 'Oxygen' is a chemical element that is vital for respiration in humans and animals, produced by plants during photosynthesis.

Wrong options:

• nutrient: 'Nutrient' does not fit as it refers to substances that provide nourishment, not a by-product of photosynthesis.
• scarcity: 'Scarcity' is unrelated as it refers to a lack of something, not a product of the photosynthesis process.
• temperature: 'Temperature' does not fit contextually as it does not describe a by-product of photosynthesis.
• sugar: 'Sugar' is a product of photosynthesis but not a by-product; it is the main product, making it incorrect in this context.
• enzyme: 'Enzyme' does not relate to the by-products of photosynthesis and is therefore irrelevant.

Correct answer: humidity

Explanation: The word 'humidity' fits grammatically as it is a noun that completes the phrase 'the amount of available light, __ levels.' Contextually, humidity is a relevant factor that can influence the efficiency of photosynthesis in plants.

Meaning: 'Humidity' refers to the amount of water vapor present in the air, which can affect plant processes including photosynthesis.

Wrong options:

• nutrient: 'Nutrient' does not fit as it does not relate to environmental factors affecting photosynthesis efficiency.
• scarcity: 'Scarcity' is unrelated as it refers to a lack of resources, not an environmental condition.
• temperature: 'Temperature' is a relevant factor but does not fit the grammatical structure of the sentence as it does not match the phrasing used.
• oxygen: 'Oxygen' is a product of photosynthesis, not a factor influencing its efficiency, making it incorrect here.
• enzyme: 'Enzyme' does not relate to environmental conditions affecting photosynthesis and is therefore irrelevant.

Correct answer: scarcity

Explanation: The term 'scarcity' fits grammatically as it is a noun that completes the phrase 'address global food __ issues.' Contextually, it is appropriate as it refers to the shortage of food resources, which is a significant concern in relation to crop yields and food production.

Meaning: 'Scarcity' refers to the state of being in short supply; a situation in which something is not easily available.

Wrong options:

• nutrient: 'Nutrient' does not fit as it does not relate to the issues of food availability or shortage.
• sugar: 'Sugar' is not relevant in this context as it does not describe a type of issue related to food availability.
• temperature: 'Temperature' does not fit as it does not describe an issue but rather a condition that can affect food production.
• oxygen: 'Oxygen' is unrelated as it does not pertain to food issues, making it incorrect in this context.
• enzyme: 'Enzyme' does not relate to food scarcity and is therefore irrelevant.