Explanation:

Main memory, also known as primary memory or RAM (Random Access Memory), is a crucial component of a computer system. It is responsible for storing data and instructions that are actively being used by the CPU (Central Processing Unit). There are various types of memory media available, but not all of them are suitable for use as main memory.

Let's analyze each option to understand why it is or isn't used as main memory:

Magnetic core:
- Magnetic core memory was one of the earliest types of random-access memory used in computers.
- It consisted of small magnetic rings or cores that could be magnetized to represent binary data.
- While magnetic core memory was used as main memory in early computer systems, it is no longer used today due to advancements in technology and the availability of more efficient memory types such as semiconductor memory.

Semiconductor:
- Semiconductor memory, also known as solid-state memory, is the most commonly used type of memory in modern computers.
- It is based on semiconductor technology and consists of integrated circuits (ICs) that store binary data in the form of electrical charges.
- Semiconductor memory is used as main memory because of its fast access times, high density, and non-volatility.
- The most common type of semiconductor memory used as main memory is Dynamic Random Access Memory (DRAM) and its variants.

Magnetic tape:
- Magnetic tape is a sequential storage medium that uses magnetic recording to store data.
- It is not suitable for use as main memory because it has slow access times and can only be accessed sequentially.
- Magnetic tape is primarily used for long-term storage and backup purposes, where large amounts of data need to be stored for archival purposes.

Conclusion:
Based on the explanations above, the memory medium that is not used as main memory is magnetic tape (option c). Magnetic core memory, although not used today, was historically used as main memory. Semiconductor memory, specifically DRAM, is the most commonly used memory type for main memory due to its speed and efficiency.