The number of tigers in the sanctuary have incresed _________________ the last 15 years.
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during
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since
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from
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for
The present perfect tense "have increased" is used here to show a change from past to present. "During" is the correct preposition for a time period that continues to the present. "Since" requires a specific point in time (e.g., "since 2009"), not a duration. "For" could work but is less idiomatic with "the last 15 years."
During is used to say something happened within a specified period of time, and "the last 15 years" names exactly that period. Since would need a fixed starting point (a date or event), from needs a paired to/until, and for states a plain duration without pointing at "the last..." phrasing the way during naturally does here. So during the last 15 years correctly frames when the increase took place.