Along with blood, water and scarlet wool, what plant did Moses use to sprinkle the people?

Answer: Hyssop

Moses took the blood of calves and goats, with water, scarlet wool and hyssop, and sprinkled the scroll and all the people.

“For when Moses had proclaimed every commandment of the law to all the people, he took the blood of calves and goats, along with water, scarlet wool, and hyssop, and sprinkled the scroll and all the people,”— Hebrews 9:19 (BSB)