Powdery materials can be guarded against caking tendency on storage by

ANS:D - none of these.

"None of these" means that none of the options provided are correct or applicable in the given context. In this case, if "none of these" is the correct answer, it indicates that the methods listed—providing irregular grain size, providing a minimum percentage of voids, and having maximum possible points of contact—are not effective in guarding against the caking tendency of powdery materials during storage. To explain further:

1. Providing irregular grain size: While irregular grain size might affect the flow properties of powdery materials, it does not necessarily prevent caking. In some cases, irregular grain size might even exacerbate caking by creating spaces where particles can become lodged or trapped.
2. Providing a minimum percentage of voids: This option seems counterintuitive because having a minimum percentage of voids would mean densifying the material, which actually helps prevent caking. So, providing a minimum percentage of voids would be an effective strategy rather than ineffective.
3. Having maximum possible points of contact: Increasing the points of contact between particles may enhance cohesion and contribute to caking rather than preventing it. Maximizing points of contact could lead to greater adhesion between particles, facilitating caking.

Therefore, if "none of these" is the correct answer, it suggests that the methods listed do not effectively guard against the caking tendency of powdery materials during storage, and other strategies or techniques not mentioned may be more suitable for this purpose.