An isobar of `""_(20)^(40)Ca` is

Options

A. 1-`._(18)^(40)Ar`
B. 2-`._(20)^(38)Ca`
C. 3-`._(1)^(1)H^(+)`
D. 4-`._(18)^(38)Ar`

Explanation

To find an isobar of \( _{20}^{40}Ca \) (Calcium-40), we need to understand the definition of isobars. Isobars are atoms that have the same atomic mass (mass number) but different atomic numbers.

### Step-by-Step Solution:

1. **Identify the mass number of Calcium-40**:
- The mass number (A) of Calcium-40 is 40. This means it has a total of 40 protons and neutrons combined.

2. **Determine the atomic number of Calcium**:
- The atomic number (Z) of Calcium is 20, which indicates that it has 20 protons.

3. **Find an element with the same mass number (40) but a different atomic number**:
- We need to look for elements that have a mass number of 40 but do not have an atomic number of 20.

4. **List possible candidates**:
- The atomic number of Argon (Ar) is 18, and it has a mass number of 40 (specifically, Argon-40).
- The atomic number of Scandium (Sc) is 21, and it also has a mass number of 40 (specifically, Scandium-40).
- Other elements like Potassium (K) and Titanium (Ti) do not have a mass number of 40.

5. **Select the correct isobar**:
- Since Argon-40 and Scandium-40 both have a mass number of 40 but different atomic numbers (18 for Argon and 21 for Scandium), they are isobars of Calcium-40.

6. **Conclude the answer**:
- Therefore, one isobar of \( _{20}^{40}Ca \) is \( _{18}^{40}Ar \) (Argon-40).

### Final Answer:
An isobar of \( _{20}^{40}Ca \) is \( _{18}^{40}Ar \) (Argon-40).