Explanation:
The correct preposition that fits in the blank is "above".

Meaning of the sentence:
The sentence describes the movement of a plane in relation to the clouds. The word "flew" indicates that the plane is in motion.

Options analysis:
Let's analyze each option to understand why "above" is the correct preposition.

1. "In": The preposition "in" is used to indicate being inside something. In this context, it does not accurately describe the plane's position in relation to the clouds.

2. "Above": The preposition "above" is used to indicate something at a higher position or level than another object. In this case, the plane is flying higher than the clouds, making "above" the appropriate preposition.

3. "Through": The preposition "through" is used to indicate movement within or between objects. While the plane might pass through the clouds, it does not accurately describe its position in relation to the clouds.

4. "To": The preposition "to" is used to indicate direction or movement towards a particular location. In this context, it does not accurately describe the plane's position in relation to the clouds.

Conclusion:
The correct preposition to fill the blank is "above" because it accurately describes the plane's position in relation to the clouds. The sentence would read: "The plane flew above the clouds."