Read the passage given below and answer the question:
The transition metals when exposed to oxygen at low and intermediate temperatures form thin, protective oxide films of up to some thousands of Angstroms in thickness. Transition metal oxides lie between the extremes of ionic and covalent binary compounds formed by elements from the left or right side of the periodic table. They range from metallic to semiconducting and deviate by both large and small degrees from stoichiometry. Since d- electron bonding levels are involved, the cations exist in various valence states and hence give rise to a large number of oxides. The crystal structures are often classified by considering a cubic or hexagonal close-packed lattice of one set of ions with the other set of ions filling the octahedral or tetrahedral interstices. The actual oxide structures, however, generally show departures from such regular arrays due in part to distortions caused by packing of ions of different size and to ligand field effects. These distortions depend not only on the number of d-electrons but also on the valence and the position of the transition metal in a period or group. (source: Smeltzer, W. W., & Young, D. J. (1975). Oxidation properties of transition metals. Progress in Solid State Chemistry, 10, 17-54.)
In the following question, a statement of assertion followed by a statement of reason is given. Choose the correct answer out of the choices on the basis of the above passage.
Assertion: Cations of transition elements occur in various valence states
Reason: Large number of oxides of transition elements are possible.

Options

A. Assertion and reason both are correct statements and reason is correct explanation for assertion.
B. Assertion and reason both are correct statements but reason is not correct explanation for assertion.
C. Assertion is correct statement but reason is wrong statement.
D. Assertion is wrong statement but reason is correct statement.

Answer

To solve the question, we need to analyze the assertion and the reason provided in the context of the passage.

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that "Cations of transition elements occur in various valence states."
- From the passage, we note that it mentions "the cations exist in various valence states" due to the involvement of d-electron bonding levels. This supports the assertion, confirming it is true.

2. **Understanding the Reason**:
- The reason states that "A large number of oxides of transition elements are possible."
- The passage explains that because cations can exist in various valence states, this leads to the formation of a large number of oxides of transition elements. Thus, this reason is also true.

3. **Evaluating the Relationship**:
- While both the assertion and reason are true, we need to determine if the reason correctly explains the assertion.
- The reason provided (large number of oxides) is a consequence of the assertion (various valence states), but it does not serve as a direct explanation for why cations exist in various valence states. Therefore, the reason does not explain the assertion correctly.

4. **Conclusion**:
- Both the assertion and reason are true statements, but the reason is not the correct explanation for the assertion. Thus, the correct choice is that both statements are true, but the reason is not the correct explanation for the assertion.

### Final Answer:
- **Assertion is true. Reason is true, but Reason is not the correct explanation for Assertion.**