Answer:
The child will most likely make the assumption that the novel word refers to the onion.
Explanation:
The human brain will always try to comprehend new information and will always try to give something a meaning.
A child's language acquisition follows a process in which he/she beings babbling, echoing what others say and beginning to form sentences and so on.
In this case, the child already knows the word book and what it is but when presented with the novel word "encyclopedia", she does not yet know what it is. When she sees the book and onion in front of her, it is likely she will think the onion is the encyclopedia.