Final answer:
The sentence presents an Ambiguous Expression due to the potential confusion about whether it is Stacy or the mountains that are 'sitting on the sidewalk'.
Step-by-step explanation:
The sentence 'Stacy said that the mountains looked beautiful sitting on the sidewalk' contains a type of expression that is B. Ambiguous Expression. An ambiguous expression is one that could have more than one meaning or that could be understood in more than one way.
In this case, the sentence may lead the reader to question whether it is Stacy or the mountains that are referred to as 'sitting on the sidewalk', which creates ambiguity. This type of confusion may occur when the components of a sentence do not clearly indicate which items they are referencing or describing.