The correct answer is option 'C' which states that the length of the straight portion of a bar beyond the end of the hook should be at least four times the diameter. Let's understand the reasoning behind this answer:

Importance of the straight portion:
The straight portion of a bar beyond the end of the hook is crucial for providing support and anchorage to the structure. It helps in transferring the load from the hook to the main body of the bar, ensuring proper stability and strength.

Reasoning:
To ensure the effectiveness and efficiency of the straight portion, it is important to have an adequate length. The length should be sufficient to provide the necessary mechanical advantage and prevent premature failure or slippage.

Considering the diameter:
The diameter of the bar is a key factor in determining the required length of the straight portion. A larger diameter bar can handle more load and requires a longer straight portion for proper support.

Comparing options:
a) Twice the diameter: This option may provide some support but may not be sufficient for handling heavy loads. It could result in reduced stability and potential failure.

b) Thrice the diameter: This option provides a better length for support, but it may still not be enough for larger loads. It is a safer choice compared to option 'a' but may not meet higher load requirements.

c) Four times the diameter: This is the correct option. Having a straight portion that is four times the diameter ensures adequate support and stability for a wide range of load requirements. It provides a suitable margin of safety and minimizes the risk of failure.

d) Seven times the diameter: This option may seem excessive. While it would provide a significant length, it may not be necessary for most applications. It could result in unnecessary material wastage and increased costs without providing substantial benefits.

Conclusion:
Considering the importance of the straight portion in providing support and anchorage, it is crucial to have a sufficient length. Option 'C', which suggests a length of four times the diameter, strikes a balance between providing adequate support and minimizing material wastage.