Which of the following is an important aspect of probability?

A) The sum of probabilities of all possible outcomes of an event equals 1.0.
B) All possible outcomes have the exact same probability of occurring.
C) Each outcome has a 50% chance of occurring.
D) Combined probabilities of events that occur at the same time is equal to 1.0.

The sum of probabilities of all possible outcomes of an event must equal 1.0, as this ensures that all possible outcomes are accounted for and that the event will occur with certainty (i.e. a probability of 1.0). This is a fundamental aspect of probability theory. B and C are incorrect, as not all outcomes necessarily have the same probability nor do they always have a 50% chance of occurring. D is also incorrect, as the combined probabilities of events that occur at the same time may not necessarily add up to 1.0 (e.g. if there is overlap in the events).