Meaning: 'Which' is used to refer to a specific thing previously mentioned, in this case, 'water power.'

Wrong options:

• what: 'What' is an interrogative pronoun that asks for information, which does not fit the context of providing additional information about 'water power.'
• In: 'In' is a preposition that indicates location or position, which does not grammatically connect to the clause about 'water power.'
• its: 'Its' is a possessive pronoun that indicates ownership, which does not fit as it does not introduce a clause.

Correct answer: up

Explanation: The word 'up' is used here as a preposition indicating the maximum amount of electricity generated, fitting grammatically and contextually within the sentence structure.

Meaning: 'Up' in this context means 'as much as' or 'to the extent of,' indicating the quantity of electricity generated.

Wrong options:

• without: 'Without' is a preposition that indicates absence, which does not fit the context of measuring electricity generation.
• but: 'But' is a conjunction used to contrast ideas, which does not fit in the context of quantifying electricity generation.
• what: 'What' is an interrogative pronoun that does not fit as it does not indicate a quantity.

Correct answer: without

Explanation: The word 'without' is a preposition that indicates the absence of emissions or pollution, fitting grammatically and contextually in the sentence.

Meaning: 'Without' means 'in the absence of' or 'not having,' indicating that the electricity generation does not produce emissions or pollution.

Wrong options:

• up: 'Up' is a preposition indicating quantity, which does not fit in the context of describing the absence of emissions.
• but: 'But' is a conjunction that introduces a contrast, which does not fit the context of describing the manner of electricity generation.
• its: 'Its' is a possessive pronoun that does not fit as it does not convey the absence of something.

Correct answer: how

Explanation: The word 'how' is an adverb that introduces a clause explaining the manner in which energy is generated, fitting grammatically and contextually in the sentence.

Meaning: 'How' refers to the method or manner of doing something, in this case, how energy is generated.

Wrong options:

• what: 'What' is an interrogative pronoun that asks for information, which does not fit the context of explaining the manner of energy generation.
• In: 'In' is a preposition that indicates location or position, which does not fit in the context of explaining the method of energy generation.
• its: 'Its' is a possessive pronoun that does not fit as it does not introduce a clause explaining the manner.

Correct answer: but

Explanation: The word 'but' is a conjunction that introduces a contrast to the previous statement about the potential of water power, fitting grammatically and contextually in the sentence.

Meaning: 'But' is used to introduce a statement that contrasts with or contradicts the previous one, indicating a limitation despite the potential.

Wrong options:

• without: 'Without' is a preposition that indicates absence, which does not fit in the context of contrasting statements.
• up: 'Up' is a preposition indicating quantity, which does not fit in the context of introducing a contrast.
• how: 'How' is an adverb that introduces a method, which does not fit in the context of contrasting the potential and the reality.