Prefixes are letters that we add to the beginning of a word to make a new word with a different meaning.
Prefixes can, for example, create a new word opposite in meaning to the word the prefix is attached to.
They can also make a word negative or express relations of time, place, or manner.
Here, in the given question the 'im' prefix is correct. Because adding 'im' before 'possible' will make a meaningful word. The meaning of it as follows:
Impossible: If an action or event is impossible, it cannot happen or be achieved
Example:
It was impossible to sleep because of the noise.
It was unpossible to sleep because of the noise.
Therefore, as per the points mentioned above, we find that the correct answer is Option 1.