Meaning: 'Limitations' means restrictions or drawbacks that prevent something from being fully effective or applicable.

Wrong options:

• certifications: 'Certifications' refers to official approvals or qualifications, which does not relate to the challenges of introducing solar panels in urban areas.
• imitations: 'Imitations' means copies or replicas, which does not convey the idea of restrictions or challenges in this context.
• implications: 'Implications' refers to possible consequences or effects, which does not directly address the constraints faced by solar panels.

Correct answer: could be

Explanation: The phrase 'could be' is appropriate here as it indicates a possibility, fitting well with the speculative nature of the statement about the use of transparent solar panels as power-generating windows.

Meaning: 'Could be' suggests that something is possible or feasible, indicating potential without certainty.

Wrong options:

• can be: 'Can be' implies certainty and ability, which is too strong for the speculative context of this sentence.
• should be: 'Should be' indicates obligation or recommendation, which does not fit the context of potential use.
• would be: 'Would be' suggests a conditional situation that is not as fitting as 'could be' in this context.

Correct answer: embedded

Explanation: The word 'embedded' fits grammatically and contextually as it describes solar cells that are integrated or placed within a solar panel, which is necessary for their function of absorbing sunlight.

Meaning: 'Embedded' means to be firmly and deeply set within something, indicating that solar cells are an integral part of the solar panel.

Wrong options:

• boarded: 'Boarded' refers to being covered or enclosed, which does not accurately describe the relationship between solar cells and panels.
• comprehended: 'Comprehended' means understood, which is irrelevant in this context as it does not relate to the physical placement of solar cells.
• deleted: 'Deleted' means removed or erased, which contradicts the idea of solar cells being necessary for power generation.

Correct answer: fraction

Explanation: The term 'fraction' is suitable as it refers to a part or portion of something, aligning with the context of how much solar light is absorbed by the devices.

Meaning: 'Fraction' means a small part or segment of a whole, indicating that these devices absorb only a portion of the solar light.

Wrong options:

• contraction: 'Contraction' refers to the act of becoming smaller or a shortened form of a word, which does not fit the context of light absorption.
• transaction: 'Transaction' refers to an exchange or transfer, which is unrelated to the concept of light absorption in solar technology.
• friction: 'Friction' refers to resistance or conflict, which does not apply to the context of solar light absorption.