To determine the approximate number of monarch butterflies the gardener will count this year, we would need to know the trend in the population of monarch butterflies over the past 20 years. However, given your question doesn't provide any specific information about the trend (whether it’s increasing or decreasing), we can't calculate a definitive answer.
If we were to assume the population has remained constant, then the answer would be 50. But if there has been a decline or increase based on current knowledge of monarch butterfly populations, the numbers may vary.
If you have details on the population trend, please share, and I can assist further! For now, based on the options provided and assuming no change, the answer would be 50.