For half an hour the glass-bottomed boat drifted. pitching and rolling above the
reef in four- to six-foot waves.
a. fragment
b. run-on
c. correct

There are actually two sentences here.  Because it is not indicated to which sentence you are referring, I will look at both.  

For half an hour the glass-bottom boat drifted.   CORRECT

Pitching and rolling above the reef in four- to six-foot waves.  FRAGMENT

The second sentence is a fragment because there is no subject within the sentence.  Even though it can be determined that what is being pitching and rolling is the boat, sentences should always be considered independently of each other.  Thus, when look at the second sentence we are not told what is doing the action.  As such, it is a fragment.  

is there an image you meant to add?

explanation:

the excerpt from chapter 2 of "animal farm", by george orwell that is an example of foreshadowing is "these three (snowball, napoleon, and squealer) had elaborated old major's teachings into a complete system of thought to which they gave the name of animalism." animal farm is an allegory of how czarist russia developed into communist russia with all its corruption. old major's teachings represent marx's theories turned into communism, animalism, by the three pigs. this foreshadows the oppression and corruption that will follow this revolt.