Question: Which of the following is not a real number?

a) √2
b) -3
c) 0
d) i

Answer: d) i

Explanation:

Real numbers are the numbers that can be represented on the number line, including negative, positive, and zero integers, fractions, and decimals. On the other hand, imaginary numbers like i do not have a place on the number line. They are represented as the square root of negative numbers, and are used in complex numbers, which are a combination of real and imaginary numbers.

To understand this better, we can break down the options:

a) √2 - This is a real number, as it can be represented on the number line as a positive irrational number.

b) -3 - This is a real number, as it can be represented on the number line as a negative integer.

c) 0 - This is a real number, as it can be represented on the number line as the origin.

d) i - This is not a real number, as it cannot be represented on the number line. It is an imaginary number, and is used in complex numbers.

Therefore, the correct option is d) i, which is not a real number.