Answer:
Lava is the correct answer
Step-by-step explanation:
In the line "Over millions of years, lava erupting from the hot spot formed a mountain", it clearly mentions that the mountain was formed due to the lava, so the rock that makes up the Hawaiian islands forms from the lava itself.
Also as the islands are mentioned as the line of volcanic mountains in the first line, it is pretty evitable that the rocks are made up of lava.