Assertion (A): Two atoms of different elements having same mass number but different atomic numbers are called isobars.
Reason (R): Atomic number is the number of protons present and mass number is the total number of protons and neutrons present in a nucleus.

Options

A. Both A and R are true and R is the correct explanation of A
B. Both A and R are true but R is NOT the correct explanation of A
C. A is true but R is false
D. A is false and R is true

Explanation

To solve the given question, we will analyze the Assertion (A) and Reason (R) separately, and then determine their validity and relationship.

### Step 1: Analyze the Assertion (A)
**Assertion (A)** states: "Two atoms of different elements having the same mass number but different atomic numbers are called isobars."

- **Definition of Isobars**: Isobars are defined as atoms of different elements that have the same mass number (A) but different atomic numbers (Z).
- **Example**: For instance, Carbon-14 (with atomic number 6) and Nitrogen-14 (with atomic number 7) are isobars because they both have the same mass number (14) but different atomic numbers.

**Conclusion for A**: The assertion is **true**.

### Step 2: Analyze the Reason (R)
**Reason (R)** states: "Atomic number is the number of protons present and mass number is the total number of protons and neutrons present in a nucleus."

- **Definition of Atomic Number**: The atomic number (Z) is indeed defined as the number of protons in the nucleus of an atom.
- **Definition of Mass Number**: The mass number (A) is defined as the total number of protons and neutrons in the nucleus (A = Z + N, where N is the number of neutrons).

**Conclusion for R**: The reason is also **true**.

### Step 3: Determine the Relationship Between A and R
- While both statements A and R are true, the reason (R) does not specifically explain the assertion (A) about isobars. The reason provides definitions of atomic number and mass number, but it does not clarify why atoms with the same mass number and different atomic numbers are called isobars.

### Final Conclusion
- Both A and R are true, but R is not the correct explanation of A.

**Final Answer**: Both A and R are true, but R is not the correct explanation of A.