Which one of the following pairs is correctly matched ?

Options

A. `{:("(a)","Mass spectrograph",-,"Chadwick"):}`
B. `{:("(b)","Atomic number",-,"Mosely"):}`
C. `{:("(c)","Neutron",-,"Milkan's"):}`
D. `{:("(d)","Measurement of charge of an electron",-,"Astron"):}`

Answer

To solve the question of which pair is correctly matched, we need to analyze each option provided:

1. **Option A: Mass Spectrograph - Chadwick**
- The mass spectrograph was not discovered by Chadwick. It was developed by William Wain. Chadwick is known for discovering the neutron.

2. **Option B: Atomic Number - Moseley**
- This option is correct. Henry Moseley is credited with the discovery of the atomic number, which is a fundamental property of elements.

3. **Option C: Neutron - Millikan**
- This option is incorrect. The neutron was discovered by James Chadwick, not Millikan. Robert A. Millikan is known for his work on the measurement of the charge of the electron.

4. **Option D: Measurement of Charge of Electron - Astron**
- This option is also incorrect. The measurement of the charge of the electron was conducted by Robert A. Millikan, not a scientist named Astron.

After analyzing all the options, we find that the only correctly matched pair is:

**Correct Answer: Option B - Atomic Number - Moseley**

### Step-by-Step Solution:
1. **Identify the scientists associated with each term in the options.**
- Mass Spectrograph: William Wain
- Atomic Number: Henry Moseley
- Neutron: James Chadwick
- Measurement of Charge of Electron: Robert A. Millikan

2. **Evaluate each option for correctness.**
- Option A: Incorrect (Chadwick did not discover mass spectrograph)
- Option B: Correct (Moseley discovered atomic number)
- Option C: Incorrect (Neutron discovered by Chadwick, not Millikan)
- Option D: Incorrect (Charge of electron measured by Millikan, not Astron)

3. **Conclude which option is correctly matched.**
- The only correct match is Option B: Atomic Number - Moseley.