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**Hint**: According to the basic definition of probability, probability of occurrence of any event is the ratio of the number of elements in the event to the total number of possible elements. Find the event in the above question. Find the total number of elements. And then use the formula of basic theorem of probability.

**:**

__Complete step-by-step answer__Let us say that $ S $ is a sample space of all possible outcomes. And $ n(S) $ is the total number of possible outcomes.

Let us say that $ E $ is an event with possible outcomes of that event. And $ n(E) $ is the total number of possible outcomes of that event.

Then, according to the basic theorem of probability, the probability of occurrence of an element in the said event is given by $ P(E) $ .

Where,

$ P(E) = \dfrac{{n(E)}}{{n(S)}} $ . . . (1)

For this question,

Sample space is all the ball. Thus, the total number of elements in the sample space is the total number of balls.

$ \Rightarrow n(S) = 9 + 7 + 4 = 20 $

And, the event is the red balls. Thus the number of elements in the event is the total number of red balls.

$ \Rightarrow n(E) = 9 $

Therefore, from equation (1), we get

$ P(E) = \dfrac{9}{{20}} $

Hence, the probability of getting a red ball is $ \dfrac{9}{{20}} $

**So, the correct answer is “ $ \dfrac{9}{{20}} $ ”.**

**Note**: In this question, knowing the basic theorem of probability and knowing how to differentiate between an event and a sample space is important. Once you understand that and can find the number of elements in the event as well as in sample space. Then this question is about just substituting the values in the formula.

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