Anumber is chosen at random from the set of numbers (21,22,,35) what is the probability that it isa multiple of 4, a prime number, the number 18, a whole number

In cases like this, the probability of a certain event is given by the ratio between the positive outcomes, and all positive outcomes. There are numbers in your set. The positive outcomes depend on the request:

There are three multples of four, namely 24,28 and 32. So, the probability of choosing one of them is

There are three prime numbers, namely 23,29 and 31. So, the probability of choosing one of them is

The number 18 doesn't belong to the set, so you can't possibly choose it. In other words, there are zero positive outcomes, so the ratio becomes

All of the numbers in the set are whole number, so you are certain that you will chose a whole number. In other words, all 15 possible outcomes are positive outcomes, so the ratio becomes

answer: your answer is a