Serpentine is a winding word. The phrase would read "The mountain road was distinctly serpentine: it twisted back and forth along the contours of the hillside" if this term were added to the text.
The commadenotes that the meaning of the omitted phrase will be clarified in the second clause of the sentence. Because a winding road does actually curve "back and forth," the word "serpentine" matches the text. The answer is not (A).
A panoramic view is one that encompasses the entire surrounding region. (B) is the wrong answer.Precipitous refers to a steep slope. The answer is not (D). The term "circumscribed" describes something that is restrained or encircled by a line. The answer is not (E). Moving backward is referred to as retrograding.
