"Sunflowers got their name not only because their blossoms are round and yellow but also because the plants are heliotropic."

What does the word "heliotropic" mean in this context? Use these dictionary entries to answer the question:
helio- (prefix) related to the sun or sunlight
-tropos (suffix) turning

plants that are warm to the touch

plants that thrive in full sun

plants that give off light

plants that swivel to face the sun