Explanation:

Prepositions:
Prepositions are words that show the relationship between a noun or pronoun and other words in a sentence. They are used to indicate location, time, direction, manner, and other relationships.

Examples of prepositions:
- at
- in
- on
- under
- beside
- between
- among

Sitting at/in/on:
The prepositions "at," "in," and "on" are commonly used to describe the position or location of someone or something. The choice of preposition depends on the context and the specific meaning you want to convey.

- "At" is used to indicate a specific point or location. It is often used with specific places like addresses or landmarks.
Example: He is sitting at the desk.

- "In" is used to indicate being inside a container, room, or enclosed space.
Example: He is sitting in the room.

- "On" is used to indicate being supported or touching the surface of something.
Example: He is sitting on the chair.

Understanding the sentence:
In the given sentence, the word "front" indicates the location of the desk in relation to the door. Since the desk is directly facing the door, we need a preposition that indicates a specific point or location. Therefore, the correct preposition to use in this sentence is "at."

Sentence breakdown:
- Mike is sitting (action)
- the desk (location)
- front of the door (specific position)

Correct answer:
Therefore, the correct answer is "Mike is sitting at the desk in front of the door." This sentence accurately conveys the positioning of Mike in relation to the desk and the door.