The type of sentence that must end with a question mark is an interrogative sentence. In this type of sentence, the speaker asks a question to gather information or seek clarification. Here is a detailed explanation of this answer:

**Interrogative Sentences:**
Interrogative sentences are used to ask questions. They are characterized by their question word order and the use of a question mark at the end. Interrogative sentences may seek information, request clarification, or express curiosity.

**Example of Interrogative Sentences:**
1. "What is your name?"
2. "Can you help me with my homework?"
3. "Where did you go on vacation?"
4. "Why are you crying?"
5. "Have you finished your dinner?"

**Function of Interrogative Sentences:**
Interrogative sentences serve the purpose of seeking information or engaging in conversation. They elicit responses from the listener or reader, encouraging them to provide an answer or share their thoughts.

**Question Mark:**
The question mark is a punctuation mark used to indicate the end of an interrogative sentence. It helps the reader or listener identify that a question is being asked. The question mark is placed at the end of the sentence, after the final word.

**Difference from Imperative and Exclamatory Sentences:**
Imperative sentences, on the other hand, give commands or make requests, and they do not end with a question mark. Examples of imperative sentences include "Please close the door" or "Pass me the salt." These sentences are used to give instructions rather than ask questions.

Exclamatory sentences express strong emotion or surprise and often end with an exclamation mark. They are used to convey excitement, astonishment, or other intense feelings. Examples of exclamatory sentences include "What a beautiful sunset!" or "I can't believe we won the game!"

**Conclusion:**
In summary, interrogative sentences are the type of sentences that must end with a question mark. They are used to ask questions and seek information. By using a question mark at the end, the sentence is clearly identified as an interrogative sentence.

Interrogative because it means question mark.