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 : Transition metals form protective oxide films.
Reason: Oxides of transition metals are always stoichiometric.

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 relation to the passage.

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states, "Transition metals form protective oxide films."
- According to the passage, it is mentioned that "the transition metals when exposed to oxygen at low and intermediate temperatures form thin, protective oxide films."
- This confirms that the assertion is **true**.

2. **Understanding the Reason**:
- The reason states, "Oxides of transition metals are always stoichiometric."
- The passage indicates that transition metal oxides "deviate by both large and small degrees from stoichiometry."
- This implies that the oxides of transition metals are **not always stoichiometric**, making the reason **false**.

3. **Conclusion**:
- Since the assertion is true and the reason is false, we can conclude that the correct answer is that the assertion is correct while the reason is incorrect.

### Final Answer:
- The correct choice based on the analysis is **Option 3**: Assertion is true, but the reason is false.