Final answer:
The difference between a number x and 7 is represented by the expression x - 7, so the correct answer is B) x - 7.
Step-by-step explanation:
The question asks to identify the mathematical expression that represents the difference between a number x and 7. The difference generally refers to the result of subtracting one number from another. So, the correct expression for calculating the difference between any number x and 7 would be found by subtracting 7 from x, which is represented as x - 7. Therefore, the answer is B) x - 7.