Answer:
1. <em><u>What</u></em> is the longest river in the United States? <u>Interrogative pronoun.</u>
2. The Missouri, <em><u>which</u></em> flows 2,540 miles, is the longest river. <u>Demonstrative pronoun.</u>
3. In fact, the Missouri is <u><em>one</em></u> of the longest rivers in the world. <u>Indefinite pronoun.</u>
4. How <em><u>many</u></em> of the world's rivers are longer than the Missouri? <u>Indefinite pronoun.</u>
Explanation:
Pronouns are the words that we use instead of a noun. There are different types of pronouns- be it demonstrative, relative, interrogative, or indefinite pronouns.
The given sentences and the type of pronouns they are, are given as below-
1. <em><u>What</u></em> is the longest river in the United States?
<u>Interrogative pronoun.</u>
2. The Missouri, <em><u>which</u></em> flows 2,540 miles, is the longest river.
<u>Demonstrative pronoun.</u>
3. In fact, the Missouri is <u><em>one</em></u> of the longest rivers in the world.
<u>Indefinite pronoun.</u>
4. How <em><u>many</u></em> of the world's rivers are longer than the Missouri?
<u>Indefinite pronoun.</u>