Question

The sun's apparent motion across the sky from East to West daily is due to the

Answer 1

Since Earth rotates on it's axis every 24 hours, only about half the planet is illuminated at one time. From the Earth, on the surface it seems that the sun is moving across the sky, but this is only a illusion due to our particularly perspective.
"The sun's apparent motion across the sky from East to West is due to the..." 
Answer: Earth's rotation.