The number of tigers in the sanctuary have incresed _________________ the last 15 years.

The correct answer is 'during' which is the best preposition to indicate a time period or duration. 'The number of tigers in the sanctuary has increased during the last 15 years' correctly uses 'during' to show that the increase happened over the course of this time period. 'During' is used with periods of time. Option B 'since' would typically require perfect tense and a specific starting point. Option C 'from' would need 'to' ('from 2008 to 2023'). Option D 'for' is used with duration but 'during' is more natural with 'the last 15 years'. Note: The sentence contains a grammar error - 'have' should be 'has' since 'number' is singular.