Meaning: 'Feat' means an impressive or remarkable achievement, especially one that requires skill or bravery.

Wrong options:

• peat: Peat is a type of soil or organic material, which does not relate to an achievement and is grammatically incorrect in this context.
• pleat: Pleat refers to a fold in fabric, which is unrelated to the context of an achievement and does not fit grammatically.
• defeat: Defeat means to lose or overcome, which is the opposite of an achievement and does not fit the positive context of the sentence.

Correct answer: region

Explanation: The word 'region' refers to a specific area, which is appropriate in the context of describing the unexplored area of the lunar south pole. It fits grammatically as a noun that describes the location being discussed.

Meaning: 'Region' means an area or division, especially part of a country or the world having definable characteristics but not always fixed boundaries.

Wrong options:

• scale: Scale refers to a system of measurement or a range, which does not fit the context of a specific area and is grammatically incorrect here.
• order: Order refers to a sequence or arrangement, which does not relate to a geographical area and is not suitable in this context.
• topic: Topic refers to the subject of discussion, which is unrelated to the geographical context of the lunar area.

Correct answer: fraction

Explanation: The word 'fraction' indicates a small part of a whole, which is fitting in the context of describing the lower cost of the moon lander compared to others. It is grammatically correct as a noun that conveys the idea of a reduced cost.

Meaning: 'Fraction' means a small part or portion of something, especially in terms of cost, indicating that the mission was achieved at a significantly lower expense.

Wrong options:

• plunge: Plunge means to dive or fall quickly, which does not relate to cost and is grammatically incorrect in this context.
• price: Price refers to the amount of money required for something, but it does not convey the idea of a reduced cost as effectively as 'fraction' does.
• peak: Peak refers to the highest point, which is unrelated to the context of cost and does not fit grammatically.

Correct answer: celebrated

Explanation: The word 'celebrated' means to acknowledge a significant achievement with joy or festivities, which fits perfectly in the context of the scientists recognizing their success. It is grammatically correct as a verb that describes their actions in response to the achievement.

Meaning: 'Celebrated' means to publicly acknowledge a significant event or achievement with joy, indicating the scientists' pride in their accomplishment.

Wrong options:

• abandoned: Abandoned means to leave behind or give up, which is the opposite of celebrating an achievement and does not fit the positive context.
• moderated: Moderated means to oversee or manage, which does not convey the idea of joy or recognition and is not suitable in this context.
• surpassed: Surpassed means to exceed or go beyond, which does not fit the context of acknowledging an achievement and is grammatically incorrect here.