Here's why:
* Derivational suffixes change the part of speech of a word or its meaning.
* -ance is a derivational suffix that typically turns a verb into a noun.
Let's break down the words:
* symbolic: This word is already a noun. It doesn't have a derivational suffix that changes its part of speech.
* satisfactory: This word is an adjective. It doesn't have a derivational suffix that makes it a noun.
* sustenance: This word is a noun. It is derived from the verb "sustain" by adding the suffix "-ance".
* strolled: This word is a verb in the past tense. It doesn't have a derivational suffix that makes it a noun.
Therefore, sustenance is the only word with a derivational suffix (-ance) that indicates it is a noun.