Which of the following statements is not correct

Options

A. Copper liberates hydrogen from acids
B. In its higher oxidation states, manganese forms stable compounds with oxygen and fluorine
C. `Mn^(3+) and Co^(3+)` are oxidising agents in aqueous solution
D. `Ti^(2+) and Cr^(2+)` are reducing agents in aqueous solution

Explanation

To determine which of the statements is not correct, we will analyze each statement regarding the properties and reactivity of the elements mentioned, particularly focusing on copper, sodium, magnesium, and their oxidation states.

### Step-by-Step Solution:

1. **Analyze the First Statement**:
- The statement suggests that copper liberates hydrogen. This is true under certain conditions, as copper can displace hydrogen from acids, but it is not a strong reducing agent compared to sodium.

**Hint**: Consider the reactivity series of metals to evaluate displacement reactions.

2. **Analyze the Second Statement**:
- This statement mentions that sodium is more reactive than magnesium. This is correct as sodium is indeed higher in the reactivity series compared to magnesium.

**Hint**: Recall the reactivity series of metals and their ability to displace other metals from compounds.

3. **Analyze the Third Statement**:
- The statement discusses the oxidation states of magnesium being more stable than those of oxygen and chlorine. This is misleading because magnesium typically exhibits a +2 oxidation state, while oxygen and chlorine can have multiple oxidation states, and their stability can vary depending on the compounds formed.

**Hint**: Compare the common oxidation states of the elements involved and their stability.

4. **Analyze the Fourth Statement**:
- The statement regarding the oxidation states of Mn, Ca, and their stability is vague and lacks clarity. However, it is known that manganese can exhibit a variety of oxidation states, and calcium typically has a stable +2 oxidation state.

**Hint**: Look into the oxidation states of transition metals and alkaline earth metals for clarity.

5. **Analyze the Fifth Statement**:
- The statement about Ti and its oxidation states is also vague. Titanium can exhibit oxidation states of +2, +3, and +4, which are relatively stable depending on the compound.

**Hint**: Investigate the common oxidation states of transition metals and their stability in various compounds.

### Conclusion:
After analyzing all the statements, the third statement regarding the stability of oxidation states of magnesium compared to oxygen and chlorine is not correct. Magnesium's +2 oxidation state is stable, but the statement implies a comparison that does not hold true universally.

### Final Answer:
The statement that is not correct is the one that claims the oxidation state of magnesium is more stable than the oxidation states of oxygen and chlorine.

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