In R.C.C. (Reinforced Cement Concrete) roofs, hooks are provided in the reinforcement bars to ensure better bonding between the steel and concrete. The length of the hook is an important factor in determining the strength and stability of the roof. The straight bar length of the hook is usually taken as a multiple of the bar diameter (D).

The correct answer to the given question is option 'C', which suggests that the straight bar length of the hook is 9 times the bar diameter.

Explanation:

1. Importance of Hooks in R.C.C. Roofs:
- R.C.C. roofs are reinforced with steel bars to provide tensile strength to the structure.
- The hooks in the reinforcement bars help in preventing the bars from slipping out of the concrete.
- They also enhance the bond between the steel and concrete, ensuring better load distribution.

2. Length of Hooks:
- The length of the hook is determined based on various factors such as bar diameter, concrete cover, and design requirements.
- The straight bar length of the hook is usually taken as a multiple of the bar diameter (D).
- The multiple value depends on the design codes and standards followed.

3. Options Provided:
a) 6D
b) 8D
c) 9D
d) 12D

4. Selection of the Correct Option:
- The correct option is 'C', which suggests that the straight bar length of the hook is 9 times the bar diameter.
- This means that if the bar diameter (D) is known, the length of the straight portion of the hook will be 9 times D.

5. Importance of the Correct Length:
- Using the correct length of hooks is essential to ensure the strength and stability of the R.C.C. roof.
- If the hook length is insufficient, the bars may slip out of the concrete, compromising the structural integrity.
- On the other hand, if the hook length is excessive, it may lead to unnecessary wastage of steel and increase construction costs.

Therefore, in R.C.C. roofs, the straight bar length of the hook is typically taken as 9 times the diameter of the bar (9D) to ensure adequate bond strength and structural stability.