`._(18)Ar^(40), ._(20)Ca^(40) and ._(19)K^(40)` are

Options

A. Isomers
B. Isotopes
C. Isobars
D. Isotones

Explanation

To solve the question regarding the nature of the elements \( _{18}^{40}\text{Ar} \), \( _{20}^{40}\text{Ca} \), and \( _{19}^{40}\text{K} \), we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Elements**:
- The notation \( _{Z}^{A}\text{X} \) represents an element with atomic number \( Z \) and mass number \( A \).
- Here, \( _{18}^{40}\text{Ar} \) is Argon with atomic number 18, \( _{20}^{40}\text{Ca} \) is Calcium with atomic number 20, and \( _{19}^{40}\text{K} \) is Potassium with atomic number 19.

2. **Check Atomic Numbers**:
- The atomic numbers are different:
- Argon (18)
- Calcium (20)
- Potassium (19)
- Since all three elements have different atomic numbers, they are different elements.

3. **Check Mass Numbers**:
- All three elements have the same mass number of 40.
- Mass number is the sum of protons and neutrons in the nucleus.

4. **Determine the Relationship**:
- Since the elements have the same mass number (40) but different atomic numbers, they cannot be isotopes (which are variants of the same element with different mass numbers).
- Instead, they are classified as isobars, which are different elements that have the same mass number.

5. **Conclusion**:
- Therefore, \( _{18}^{40}\text{Ar} \), \( _{20}^{40}\text{Ca} \), and \( _{19}^{40}\text{K} \) are isobars.

### Final Answer:
The elements \( _{18}^{40}\text{Ar} \), \( _{20}^{40}\text{Ca} \), and \( _{19}^{40}\text{K} \) are isobars.

---