Final answer:
Among the given options, 'croon' serves as the synonym for 'soothe' in the context of the excerpt from 'The People Could Fly', indicating a soft, calming singing or humming.
Step-by-step explanation:
In the excerpt from The People Could Fly, the word that provides a definition for "soothe" among the options is croon. 'Croon' refers to the act of singing or humming softly in a soothing manner, typically to calm someone down. This can be deduced from the context wherein the character refrains from soothing the crying child, without the heart to croon to it, suggesting that 'croon' is a synonym of 'soothe' in this scenario.