Meaning: 'Pioneering' refers to work that is original and leads the way for others in a particular field.

Wrong options:

• reciprocal: The term 'reciprocal' refers to something that is mutual or given in return, which does not fit the context of describing Curie's work.
• tense: The word 'tense' is a noun that relates to time in grammar, which does not apply to the description of Curie's work.
• comparatively: 'Comparatively' is an adverb used to compare two things, which does not appropriately describe the nature of her work.

Correct answer: practically

Explanation: The adverb 'practically' indicates that Curie applied her research in a practical manner during World War I, fitting grammatically and contextually as it modifies the verb 'applied.' It emphasizes the real-world application of her scientific findings.

Meaning: 'Practically' means in a way that is concerned with actual use or practice rather than theory.

Wrong options:

• reciprocal: 'Reciprocal' does not fit here as it does not relate to the application of research in a practical sense.
• tense: 'Tense' is unrelated to the context of applying research and does not fit grammatically as an adverb here.
• comparatively: 'Comparatively' does not apply to the manner of application and does not fit the context.

Correct answer: prolonged

Explanation: The adjective 'prolonged' describes the duration of Curie's exposure to radiation, fitting grammatically as it modifies 'exposure.' It conveys the seriousness of her health issues resulting from extended exposure, aligning with the context of the sentence.

Meaning: 'Prolonged' means extended in time or duration, indicating that her exposure to radiation was significant and lasting.

Wrong options:

• reciprocal: 'Reciprocal' does not relate to the duration of exposure and is not appropriate in this context.
• tense: 'Tense' is a noun that does not apply to the description of exposure and does not fit grammatically.
• comparatively: 'Comparatively' does not describe the nature of exposure and is not relevant here.

Correct answer: countless

Explanation: The adjective 'countless' describes the number of future scientists inspired by Curie's legacy, fitting grammatically as it modifies 'scientists.' It emphasizes the vast impact of her contributions, aligning with the positive sentiment of the sentence.

Meaning: 'Countless' means too many to be counted; innumerable, indicating a large number of future scientists.

Wrong options:

• reciprocal: 'Reciprocal' does not fit as it refers to mutual relationships, which does not apply to the number of scientists inspired.
• tense: 'Tense' does not relate to the quantity of scientists and is not appropriate in this context.
• comparatively: 'Comparatively' does not describe the number of scientists and does not fit the context.