Bohr’s theory is applicable to:

Options

A. `He`
B. `Li^(2+)`
C. `He^(2+)`
D. None of these

Explanation

To determine the applicability of Bohr's theory, we need to identify the species that have only one electron. Bohr's theory is specifically designed for single-electron systems. Let's analyze the given options step by step.

### Step 1: Understand Bohr's Theory
Bohr's theory is applicable only to single-electron species. This means we need to look for atoms or ions that have exactly one electron.

**Hint:** Remember that Bohr's model is primarily used for hydrogen-like atoms or ions, which have one electron.

### Step 2: Analyze the Given Options
We need to check the number of electrons in each species provided in the question.

1. **Hydrogen (H)**:
- Atomic number = 1
- Number of electrons = 1 (Bohr's theory is applicable)

2. **Helium (He)**:
- Atomic number = 2
- Number of electrons = 2 (Bohr's theory is NOT applicable)

3. **Lithium ion (Li²⁺)**:
- Atomic number of Lithium = 3
- Lithium has 3 electrons. Since it has a +2 charge, it has lost 2 electrons.
- Number of electrons = 3 - 2 = 1 (Bohr's theory is applicable)

4. **Helium ion (He²⁺)**:
- Atomic number of Helium = 2
- Helium has 2 electrons. With a +2 charge, it has lost both electrons.
- Number of electrons = 2 - 2 = 0 (Bohr's theory is NOT applicable)

### Step 3: Conclusion
From the analysis, we find that:
- Hydrogen (H) has 1 electron, so Bohr's theory is applicable.
- Lithium ion (Li²⁺) has 1 electron after losing 2, so Bohr's theory is applicable.
- Helium (He) has 2 electrons, so Bohr's theory is NOT applicable.
- Helium ion (He²⁺) has 0 electrons, so Bohr's theory is NOT applicable.

Thus, the only species from the options that Bohr's theory is applicable to is **Li²⁺**.

### Final Answer
Bohr's theory is applicable to **Li²⁺**.

---