Answer:
to hurl
Step-by-step explanation:
Infinitives are verbals which are the words made from verbs but functioning as something else, noun, adjective, adverb etc.). The verbals are of three types infinitives (to hurl a stone, to fly is interesting), gerunds (smoking kills), participles (he is a learned man, The dead plants).
Verbals made by adding "to" before first form of verb are called infinitives e.g To swim in the river requires skill, He like to study at night.
In the sentence provided "to hurl stones" is a noun phrase, acting as direct subject.