Read the following sentence, paying close attention to the underlined words:
the young child played alone in the warm sun while the older children swam in the cold water of the clear lake nearby.
which underlined word does not function as an adjective?
a. warm
b. clear
c. young
d. alone

In this sentence...
"Warm" is describing the sun. 
"Clear is describing the water. 
"Young is describing the child.
but here "alone" is more so the state of being...it describes how she played, which we call an adverb. 

An Adverb- just describes another verb, adjective or adverb. Shows time, manner or place. (In this case it is showing manner) Here "alone"tells us how she played.