During a negative beta decay,

Options

A. (a) an atomic electron is ejected
B. (b) an electron which is already present within the nucleus is ejected
C. (c) a neutron in the nucleus decays emitting an electron
D. (d) a part of the binding energy of the nucleus is converted into a electron

Explanation

### Step-by-Step Solution:

1. **Understanding Negative Beta Decay**:
- Negative beta decay is a type of radioactive decay in which a neutron in the nucleus of an atom transforms into a proton, emitting an electron (beta particle) and an antineutrino.

2. **Identifying the Particles Involved**:
- In this process, the neutron (n) decays into a proton (p), an electron (e⁻), and an antineutrino (ν̅). The reaction can be represented as:
\[ n \rightarrow p + e^- + \nu̅ \]

3. **Analyzing the Options**:
- Option A: "Atomic electron is ejected" - This is incorrect because the electron emitted is not an atomic electron; it is a beta particle produced from the decay.
- Option B: "An electron which is already present within the nucleus is ejected" - This is incorrect because there are no electrons present in the nucleus.
- Option C: "A neutron in the nucleus decays and emits an electron" - This is correct as it describes the process accurately.
- Option D: "A part of binding energy of the nucleus is converted into electron" - This is misleading; while energy conservation applies, the decay process itself is not about converting binding energy into an electron.

4. **Conclusion**:
- The correct answer is Option C: "A neutron in the nucleus decays and emits an electron."