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Which of the following is an irrational number?

  1. 9

  2. √2

  3. 0.15

  4. -3

The correct answer is: √2

An irrational number is a number that cannot be expressed as a fraction or a terminating decimal. Option A is incorrect because 9 can be written as a fraction 9/1. Option C is incorrect because 0.15 is a terminating decimal (it has a finite number of digits after the decimal point). Option D is incorrect because -3 can be written as a fraction -3/1. Option B, √2, is an irrational number because it cannot be written as a fraction and it is a non-terminating decimal (it has an infinite number of digits after the decimal point). Therefore, option B is the only correct answer.