Which of the following cannot be emitted by radioactive substances during their decay ?

Options

A. Neutrinos
B. Protons
C. Electrons
D. Helium nuclei

Explanation

To determine which of the following cannot be emitted by radioactive substances during their decay, we will analyze each option provided: neutrinos, protons, electrons, and helium nuclei.

### Step 1: Understand the types of radioactive decay
Radioactive decay can occur in several forms, including:
- **Alpha decay**: Emission of helium nuclei (alpha particles).
- **Beta decay**: Emission of electrons (beta particles) and neutrinos.
- **Gamma decay**: Emission of gamma rays, which are high-energy photons.

### Step 2: Analyze each option
1. **Neutrinos**:
- Neutrinos are emitted during beta decay. Therefore, they can be emitted by radioactive substances.

2. **Protons**:
- Protons are not typically emitted during any form of radioactive decay. They are constituents of the nucleus and are not released as particles in decay processes.

3. **Electrons**:
- Electrons are emitted during beta decay (specifically beta minus decay). Thus, they can be emitted by radioactive substances.

4. **Helium nuclei**:
- Helium nuclei (alpha particles) are emitted during alpha decay. Therefore, they can also be emitted by radioactive substances.

### Step 3: Conclusion
After analyzing the options, we find that:
- Neutrinos, electrons, and helium nuclei can be emitted during radioactive decay.
- Protons cannot be emitted during radioactive decay.

Thus, the answer is that **protons cannot be emitted by radioactive substances during their decay**.

### Final Answer:
**Protons cannot be emitted by radioactive substances during their decay.**

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