The best paraphrase of the excerpt is:
"For more than 20 years, thousands of volunteers have hiked trails across Ohio with a purpose in mind. Between April and October, as they walk on the trail they have chosen, they identify and count every butterfly they can find within a certain distance. They then send their butterfly data to a nonprofit group called The Ohio Lepidopterists, who conserve and appreciate butterflies, skippers, and moths."
This option accurately captures the key elements of the original text, including the duration of the project, the role of the volunteers, the specific actions they take, and the purpose of their data collection.