Explanation
The most efficient subnet address for a network that will have no more than 30 available IP addresses is
255.255.255.224. This subnet mask corresponds to a /27 prefix length, which means that 27 bits are used for the network portion and 5 bits are used for the host portion of an IP address. With 5 bits for hosts, there are
2^5 - 2 = 30 possible host addresses per subnet, which meets the requirement. The other options are either too large or too small for the network size. Reference: https://www.ibm.com/cloud/learn/subnet-mask

RAID 6 Disk Striping with Double Parity
The RAID 6 design distributes data across a minimum of four HDDs the same way that RAID 0 and RAID 5 do, but it also distributes parity information across two disks. The result is that a RAID 6 array can recover data even with the failure of two HDDs. Reads are quick, like RAID 5, but writes are slower due to the duplication of the parity data. It is a good general solution as long as the performance hit on write tasks is not a problem for your environment.
RAID 6 requires a minimum of four HDDs.

Explanation
The best course of action to preserve business continuity for a very old PC that is running a critical, proprietary application in MS-DOS is to purchase matching hardware and clone the disk. This way, the technician can create an exact copy of the PC's configuration and data on another PC that has the same specifications and compatibility. This will ensure that the application can run smoothly on the new PC without any installation or configuration issues. Performing scheduled chkdsk tests would not help, as chkdsk is a tool that checks and repairs disk errors, but does not prevent hardware failures or software compatibility issues.
Upgrading the hard disk to SSD would not help either, as SSDs may not be compatible with the old PC or the MS-DOS operating system. Performing quarterly backups would help with data protection, but not with hardware availability or software compatibility. References:
https://www.howtogeek.com/199068/how-to-upgrade-your-existing-hard-drive-in-under-an-hour/https://www.ho

Explanation
Grandfather-father-son (GFS) is a common backup rotation scheme that uses daily, weekly, and monthly backup media in a first-in, first-out (FIFO) method. The daily backups are rotated on a 3-months basis using a FIFO system as above. The weekly backups are similarly rotated on a bi-yearly basis, and the monthly backups are rotated on an annual basis. The oldest backup media in each cycle are overwritten by the newest ones. This scheme provides multiple versions of backup data at different intervals, allowing for flexible restoration options. Waterfall is another name for GFS. Synthetic full is a backup method that combines an initial full backup with subsequent incremental backups to create a new full backup without transferring all data again. Tower of Hanoi is another backup rotation scheme that uses an algorithm based on moving disks between three pegs. References:
https://en.wikipedia.org/wiki/Backup_rotation_scheme