Meaning: 'Uneconomic' means not profitable or not worth the expense involved in obtaining something.

Wrong options:

• rebate: A 'rebate' is a return of part of a payment, which does not fit the context of describing fuel sources.
• renew: The word 'renew' is a verb that means to make new again, which does not appropriately describe the state of fuel sources in this context.
• suspect: 'Suspect' is a verb meaning to have doubts about something, which does not fit grammatically or contextually in this sentence.
• fossil: 'Fossil' is an adjective that describes fuels derived from ancient organic matter, which does not fit the context of describing the economic viability of fuel sources.
• constitute: 'Constitute' is a verb meaning to make up or form something, which does not relate to the economic aspect of fuel sources.

Correct answer: constitute

Explanation: The verb 'constitute' fits grammatically as it indicates that renewable energy sources make up a certain percentage of electricity usage. It aligns with the context of quantifying the contribution of renewable energy to the overall electricity supply.

Meaning: 'Constitute' means to make up or form a part of something.

Wrong options:

• rebate: 'Rebate' is a noun that refers to a return of part of a payment, which does not fit the context of describing the percentage of electricity.
• renew: 'Renew' is a verb that means to make something new again, which does not fit the grammatical structure needed here.
• suspect: 'Suspect' is a verb that means to doubt or question something, which does not fit in the context of describing a percentage of electricity.
• fossil: 'Fossil' is an adjective that describes certain types of fuels, which does not fit the context of quantifying energy sources.
• uneconomic: 'Uneconomic' is an adjective that describes something not profitable, which does not fit the context of stating a percentage.

Correct answer: fossil

Explanation: The word 'fossil' fits grammatically as it describes a type of fuel, specifically fossil fuels, which contrasts with renewable energy sources. This contextual distinction is crucial for understanding the environmental impact of different energy types.

Meaning: 'Fossil' refers to fuels derived from ancient organic matter, such as coal, oil, and natural gas.

Wrong options:

• rebate: 'Rebate' is a noun that does not relate to types of fuels and does not fit the context of the sentence.
• uneconomic: 'Uneconomic' is an adjective that describes a lack of profitability, which does not fit as a descriptor for fuel types.
• constitute: 'Constitute' is a verb that means to make up or form something, which does not fit as a descriptor for fuel types.
• renew: 'Renew' is a verb that means to make something new again, which does not fit the context of describing fuel types.
• suspect: 'Suspect' is a verb that means to doubt or question, which does not fit in the context of describing fuel types.