Anecdotal evidence means that is an observation made casually, without keeping track systematically.
Options II, III, and IV are statements that draw conclusions from data collected over the years, such as the average temperature, the rate of fish spawning, and the level of precipitation, so they are not anecdotal evidence.
But the observation that some flowers are blooming earlier every year, without any more data (such as a date range), is a casual observation, and so is anecdotal evidence.
Hence, I. Some flowers are blooming earlier every year, is the right answer.