Explanation:
To explain why the correct answer is "in," we need to understand the relationship between the verb "live" and the preposition that follows it. In this case, the verb "live" implies that the subject is residing or dwelling somewhere. The preposition that typically follows "live" is "in," which denotes the location or place where someone lives. Here are the reasons why "in" is the appropriate choice:
- Grammatical usage: The verb "live" is usually followed by the preposition "in" to indicate the location of residence. Using "at" or "on" in this context would be grammatically incorrect.
- Idiomatic expression: "To live in" is an idiomatic expression commonly used to describe the act of residing or occupying a particular place.
- Comfort: The sentence states that the house is comfortable to live in, emphasizing the comfort one experiences while residing inside the house.
Therefore, the correct phrase to complete the sentence is "This is a comfortable house to live in."

Living in a House
Living "in" a house is the correct preposition to use because it implies being inside the physical structure and making it your home. Here are some reasons why this preposition is more suitable:

Comfort and Security
- When you live in a house, you are surrounded by walls and a roof that provide comfort and security.
- You have a space that is designated as your own, where you can relax, unwind, and feel safe.

Community and Neighborhood
- Living in a house often means being part of a community or neighborhood, where you can interact with neighbors and build relationships.
- You have the opportunity to engage in local events, participate in community activities, and feel connected to your surroundings.

Personalization and Ownership
- When you live in a house, you have the freedom to personalize and decorate your living space according to your preferences.
- You can make changes to the interior and exterior of the house to reflect your style and create a place that feels like home.

Conclusion
In conclusion, living "in" a house offers a sense of comfort, security, community, and personalization that makes it a desirable place to call home. It provides a space where you can truly feel at ease and create lasting memories with your loved ones.