Final answer:
The expression that represents multiplying a number by itself is 'x²', also known as squaring the number, and the correct answer is option D) x².
Step-by-step explanation:
The expression that represents the event of multiplying a number by itself is x². When we use an exponent, such as the number "2" in this case, it means to multiply the base number, "x", by itself. This is known as "squaring" the number, which can also be represented as x × x. None of the other options provided, such as y × x, x ÷ y, or y × x correctly represent the squaring of a number. The correct answer is D) x².