The correct answer is option 'D' - none of the listed. Let's analyze each option to understand why it is not the correct answer.

a) Increasing the shank diameter:
Increasing the shank diameter of a bolt does not necessarily improve its shock absorbing capacity. The diameter of the shank affects the strength and load-carrying capacity of the bolt, but it does not directly impact its ability to absorb shock.

b) Decreasing the length of shank portion of bolt:
Similarly, decreasing the length of the shank portion of a bolt does not directly enhance its shock absorbing capacity. The length of the shank is primarily determined by the specific application requirements and the structural design considerations. Changing the length of the shank does not affect the bolt's ability to absorb shock.

c) Increase the shank diameter and decreasing the length of shank:
Although increasing the shank diameter and decreasing the length of the shank may result in a stronger bolt, it does not necessarily improve its ability to absorb shock. These changes primarily affect the bolt's load-carrying capacity and resistance to shear and tension forces, but they do not directly enhance its shock absorption capability.

Therefore, none of the listed options are correct for increasing the shock absorbing capacity of bolts.

Explanation:

To enhance the shock absorbing capacity of bolts, other factors need to be considered, such as the material properties, design features, and the presence of additional components. Some of the factors that can influence the shock absorbing capacity of bolts include:

1. Material selection:
Choosing a material with high impact resistance, such as alloy steel or titanium, can improve the shock absorbing capacity of bolts. These materials have better resilience and toughness, allowing them to absorb and dissipate shock energy effectively.

2. Thread design:
The thread design of a bolt can also contribute to its shock absorbing capacity. Deep threads and fine pitches provide better grip and resistance against loosening due to vibrations or sudden impacts.

3. Use of washers:
Adding washers, such as Belleville washers or spring washers, between the bolt head/nut and the connected surfaces can help improve shock absorption. These washers act as springs, absorbing and distributing the shock load over a larger area.

4. Proper tightening:
Ensuring that the bolts are properly tightened to the recommended torque or tension helps to maintain their shock absorbing capacity. Under or over-tightening can negatively affect the bolt's ability to absorb shock.

5. Structural design considerations:
The overall structural design, including the use of additional supports or dampening elements, can also play a significant role in enhancing the shock absorbing capacity of bolts. By minimizing the transmission of shock waves or vibrations to the bolted joints, the bolts can experience less stress and better absorb the shocks.

In conclusion, increasing the shank diameter or decreasing the length of the shank does not directly improve the shock absorbing capacity of bolts. Other factors such as material selection, thread design, use of washers, proper tightening, and structural design considerations need to be taken into account to enhance the shock absorbing capacity of bolts.