Which part of this sentence is literal

stars rocketed across the sky that night, as if pulled on a string held by a giant.

a. across the sky
b. stars rocketed
c. pulled on a string
d. held by a giant

The best answer would be letter A. Across the sky

Among all of the given choices, it's the phrase that could be interpreted in the most literal sense. 

Option B- Stars rocketed could be literally imagined but it can be a form of hyperbole. It can be an exaggerated expression. 

Option C- This is also another phrase that could have a literary device.

Option D- Another figure of speech

So, Option A is really the correct answer.