The maximum number of primary partitions that can be created on a hard disk is 4.

Explanation:
1. What is a primary partition?
A primary partition is a section of a hard disk that functions as a standalone storage unit. Each primary partition can be formatted with a file system and used to store data or install an operating system.

2. Why is the maximum number of primary partitions limited to 4?
The limitation of 4 primary partitions is due to the structure of the Master Boot Record (MBR) partitioning scheme. MBR is an older partitioning method used on traditional BIOS-based systems.

3. MBR partitioning scheme:
- MBR divides the hard disk into four primary partitions or three primary partitions and one extended partition.
- The extended partition can be further divided into logical partitions, which are additional storage units within the extended partition.
- However, only the primary partitions can be used to boot an operating system.
- The extended partition is a workaround to overcome the limitation of 4 primary partitions.

4. Partitioning options:
a) With 4 primary partitions:
- You can create 4 primary partitions on a hard disk.
- Each primary partition can be used independently and contain its own file system.
- This is the maximum number of primary partitions allowed by the MBR partitioning scheme.

b) With 3 primary partitions and 1 extended partition:
- If you choose to create an extended partition, you can have a maximum of 3 primary partitions and 1 extended partition.
- Within the extended partition, you can create multiple logical partitions.
- The logical partitions function similarly to primary partitions and can also be formatted with a file system.

5. Other partitioning schemes:
For systems that require more than 4 primary partitions, other partitioning schemes can be used, such as GUID Partition Table (GPT) or using multiple hard disks.

In conclusion, the MBR partitioning scheme allows a maximum of 4 primary partitions on a hard disk. Having more than 4 primary partitions would require alternative partitioning methods.