Concept:A book is made of pages, so the analogy follows a whole to part relationship.
Explanation:A book is formed by many pages.
Pages are the smaller units that together make a book.
Using the same relation, a continent is formed by many countries.
Countries are the smaller units that together make a continent.
Water, forest, and land are not the direct constituent units in this analogy.
So the missing word is countries.
Answer:Countries