Based on the available information and records, the event that happened recently is "the number of snow leopards in Sailugemsky National Park reached 15."
The available events and the year of occurrence are the following:
- Mergen Markov returned home to the village of Argut - It occurred in the year 2000
- The Soviet Union collapsed and poaching began to increase in Russia - It occurred in 1991
- Sailugemsky National Park was established as a protected area for snow leopards - it occurred in 2010
- The number of snow leopards in Sailugemsky National Park reached 15 - It occurred in 2020
Hence, in this case, it is concluded that the correct answer is option D.
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