Which of the following exhibits only +3 oxidation state?

Options

A. Th
B. Ac
C. Pa
D. U

Explanation

To determine which of the given elements exhibits only a +3 oxidation state, we will analyze each element's electronic configuration and possible oxidation states.

### Step 1: Analyze Thorium (Th)
- **Atomic Number**: 90
- **Electronic Configuration**: Rn 6d² 7s²
- **Possible Oxidation States**:
- Thorium can lose 2 electrons from the 7s orbital and 2 electrons from the 6d orbital, leading to oxidation states of +2, +3, and +4.

### Step 2: Analyze Actinium (Ac)
- **Atomic Number**: 89
- **Electronic Configuration**: Rn 6d² 7s¹
- **Possible Oxidation States**:
- Actinium primarily exhibits a +3 oxidation state as it can lose the single 7s electron and 2 electrons from the 6d orbital. Thus, it predominantly shows +3.

### Step 3: Analyze Protactinium (Pa)
- **Atomic Number**: 91
- **Electronic Configuration**: Rn 5f² 6d¹ 7s²
- **Possible Oxidation States**:
- Protactinium can lose electrons from the 7s and 6d orbitals, leading to oxidation states of +3, +4, and +5.

### Step 4: Analyze Uranium (U)
- **Atomic Number**: 92
- **Electronic Configuration**: Rn 5f³ 6d¹ 7s²
- **Possible Oxidation States**:
- Uranium can exhibit oxidation states of +3, +4, +5, and +6 by losing electrons from the 5f, 6d, and 7s orbitals.

### Conclusion
After analyzing the oxidation states of all four elements, we find that only **Actinium (Ac)** exhibits only a +3 oxidation state.

### Final Answer
**Actinium (Ac)** exhibits only +3 oxidation state.

---