Meaning: 'Overcoming' means successfully dealing with or gaining control over something difficult, in this case, the challenges faced by perovskite solar cells.

Wrong options:

• mediating: Mediating implies acting as an intermediary, which does not fit the context of addressing challenges directly.
• abhorring: Abhorring means to regard with disgust or hatred, which is irrelevant to the context of scientific progress.
• observing: Observing refers to watching or monitoring, which does not convey the action of resolving challenges.

Correct answer: limited

Explanation: The word 'limited' is the correct past participle form that fits the sentence structure, indicating a completed action affecting the practical application of perovskite solar cells. It aligns with the past context of the challenges faced by these materials.

Meaning: 'Limited' means restricted in extent or scope, indicating that the challenges previously restricted the application of perovskite solar cells.

Wrong options:

• limit: Limit is the base form of the verb and does not fit grammatically in this past context.
• have limited: Have limited suggests an ongoing action, which contradicts the past context of the challenges faced.
• would have limited: Would have limited implies a hypothetical situation that did not occur, which is not applicable here.

Correct answer: forefront

Explanation: The word 'forefront' fits contextually as it indicates being at the leading position in research, which aligns with the advancements made in perovskite materials. It also matches the tone of progress and innovation in the field.

Meaning: 'Forefront' refers to the leading or most important position in a particular field, indicating that perovskite materials are now leading in solar energy research.

Wrong options:

• backdrop: Backdrop refers to a background setting, which does not convey the idea of leadership in research.
• appendix: Appendix refers to supplementary material, which is not relevant to the context of being at the forefront of research.
• foundation: Foundation implies a base or groundwork, which does not capture the idea of being at the leading edge of research.

Correct answer: impeded

Explanation: The word 'impeded' is the correct past participle that fits the sentence structure, indicating that past issues have obstructed the practical application of perovskite solar cells. It aligns with the context of challenges faced in the past.

Meaning: 'Impeded' means hindered or obstructed, indicating that previous challenges have slowed down the practical application of these solar cells.

Wrong options:

• cited: Cited means mentioned or referred to, which does not fit the context of hindering application.
• identified: Identified means recognized or discovered, which does not convey the idea of obstruction.
• monitored: Monitored means observed or kept track of, which does not relate to the concept of hindering application.