Meaning: 'Reached' means to arrive at or attain a particular point or level.

Wrong options:

• solid: 'Solid' is an adjective that describes a state of matter or firmness, which does not fit the context of achieving a peak in engineering.
• satisfied: 'Satisfied' is an adjective that indicates contentment, which does not relate to the action of reaching a zenith in engineering.
• supposed: 'Supposed' is a past participle that implies assumption or expectation, which does not convey the action of achieving a peak.
• attributed: 'Attributed' is a verb that means to regard something as being caused by someone or something, which does not fit the context of reaching a zenith.
• coincided: 'Coincided' is a verb that means to occur at the same time, which does not relate to the achievement of a peak in engineering.
• waning: 'Waning' is a verb that means to decrease in size or strength, which contradicts the idea of reaching a zenith.

Correct answer: attributed

Explanation: The word 'attributed' is used to indicate that the fall of the empire is ascribed to the overstretching of resources. It fits grammatically as it follows the passive construction of the sentence, maintaining clarity and coherence.

Meaning: 'Attributed' means to regard something as being caused by someone or something.

Wrong options:

• solid: 'Solid' does not fit in this context as it is an adjective and does not convey the action of ascribing causes.
• satisfied: 'Satisfied' is an adjective that does not relate to the act of attributing causes to events.
• supposed: 'Supposed' implies assumption rather than attribution, which does not fit the context of explaining the cause of the empire's fall.
• reached: 'Reached' is a verb that indicates achieving a point, which does not relate to the act of attributing causes.
• coincided: 'Coincided' suggests simultaneous occurrence, which does not fit the context of attributing a cause to an event.
• waning: 'Waning' refers to a decrease, which does not relate to the act of attributing causes.

Correct answer: coincided

Explanation: The verb 'coincided' indicates that the decrease in construction projects occurred simultaneously with the period of decline. This fits grammatically and contextually, as it describes the relationship between two events.

Meaning: 'Coincided' means to occur at the same time as something else.

Wrong options:

• solid: 'Solid' is an adjective that does not convey the action of events occurring simultaneously.
• satisfied: 'Satisfied' is an adjective that does not relate to the timing of events.
• supposed: 'Supposed' does not fit as it implies assumption rather than simultaneous occurrence.
• reached: 'Reached' indicates achieving a point, which does not relate to the timing of events.
• attributed: 'Attributed' refers to ascribing causes, which does not fit the context of simultaneous events.
• waning: 'Waning' refers to a decrease, which does not convey the idea of coinciding events.

Correct answer: waning

Explanation: The term 'waning' describes a gradual decrease in the empire's ability to sustain its infrastructure. It fits grammatically as it describes the diminishing capability, aligning with the context of decline.

Meaning: 'Waning' means to decrease in size, strength, or intensity.

Wrong options:

• solid: 'Solid' is an adjective that does not convey the idea of decrease in ability or strength.
• satisfied: 'Satisfied' is an adjective that indicates contentment, which does not relate to the concept of diminishing capability.
• supposed: 'Supposed' implies assumption rather than a decrease in ability, which does not fit the context.
• reached: 'Reached' indicates achieving a point, which contradicts the idea of a decline in ability.
• attributed: 'Attributed' refers to ascribing causes, which does not fit the context of describing a decrease in ability.
• coincided: 'Coincided' refers to simultaneous occurrence, which does not relate to the concept of waning ability.