Answer
(a) This event is impossible. It can never occur. 0
(b) This event is certain. It will occur in every trial. 1
(c) This event is very likely, but it will not occur once in a while in a long sequence of trials. 0.99
(d) This event will occur slightly less often than not. 0.45
Work Step by Step
Probability is a measure of how likely something is to happen. It can be written in many forms, though the easiest to understand is as a fraction. The probability of an event being $a/b$ means that for every $b$ trials, the event will occur $a$ times. Converting this to a decimal number yields equivalent answers.
(a) If an event is impossible, it will occur $0$ out of $a$ times, or have probability $0/a = 0$
(b) If an event must happen every time, it will occur $a$ out of $a$ times, or have probability $a/a = 1$
(c) If an event is very likely, but will not occur once in a while in a long sequence of trials, the probability is very close to guaranteed (1). The option that best fits this is $0.99$
(d) If an event occurs slightly less often than not, it occurs slightly less than half of the time. A half probability is 0.5, and the option that is close but less than this is 0.45
