Probability always sits somewhere on a fixed scale between 0 and 1, where the two extremes represent the two "sure" outcomes of chance, never happening and always happening.
An event that has no way of occurring, since it isn't even part of the possible outcomes, sits at the very bottom of that scale.
Think of tossing a standard six-sided die: the event "getting a 7" isn't a possible outcome at all, so no matter how many times you repeat the experiment, it occurs 0 times out of the total trials. The ratio of favorable to total outcomes is therefore \( \frac{0}{n} = 0 \) for any \( n \).
This is the opposite end of the scale from a certain event, which occurs every trial and has a ratio of \( \frac{n}{n} = 1 \). An impossible event occupies the other extreme of that same scale.
So the correct answer is 0.