On reaching Dronagiri Lord Hanuman got confused what to pick as there were a large varieties of herbs on the mountain. He carried the entire mountain and placed it before Sushena. On being given the herb, Lord Lakshman recovered from his injuries in a short span of time.
Science, as a reference has listed Selaginella Bryopteris as the Sanjeevani that might have been used as the buti or medicine during the period. However, this hasn't been proved yet, therefore, it's difficult to say what it really is.(Also read: Dussehra 2017: Significance, Date and Rituals)
The mystery of Sanjeevani buti is far from solved considering scientists are still working on it and nobody has till date been able to really find out what herbs were actually used. But, a herb like this would definitely have the power to cure many with its magical properties.