Answer:
The symbol of an element X, given its mass number A and atomic number Z, is represented as AZX. Let's break down the symbol to understand why option 'B' is the correct answer.

Understanding the symbol:
- A represents the mass number of the element, which is the sum of the protons and neutrons in the nucleus of an atom.
- Z represents the atomic number of the element, which is the number of protons in the nucleus of an atom.
- X represents the chemical symbol of the element, which is a one or two-letter abbreviation used to represent an element.

Explanation of the correct answer:
Option 'B' is the correct answer because it follows the correct order of the elements in the symbol.

- A is the mass number, which represents the total number of protons and neutrons in the nucleus of the atom. It should come before Z.
- Z is the atomic number, which represents the number of protons in the nucleus of the atom. It should come after A.
- X is the chemical symbol, which represents the element. It should come at the end.

Therefore, the correct symbol of the element X, given its mass number A and atomic number Z, is AZX.

Explanation of incorrect options:
- Option 'A' (AXZ) is incorrect because it does not follow the correct order of the elements in the symbol. The mass number should come first, followed by the atomic number, and then the chemical symbol.
- Option 'C' (ZAX) is incorrect because it reverses the order of the elements in the symbol. The atomic number should come before the chemical symbol.
- Option 'D' (ZXA) is incorrect because it also reverses the order of the elements in the symbol. The atomic number should come before the chemical symbol.

Conclusion:
The correct symbol of an element X, given its mass number A and atomic number Z, is represented as AZX. It is important to follow the correct order of the elements in the symbol to accurately represent the element.