Introduction:
Magnetic memory is a type of computer memory that uses magnetization to store and retrieve data. It is commonly used in various storage devices such as hard disks, tapes, and magnetic cores. This question asks to identify which option is not a magnetic memory.

Explanation:
Let's discuss each option and determine whether it is a magnetic memory or not.

1. Core:
A magnetic core memory is a type of random-access memory (RAM) that uses tiny magnetized rings or cores to store information. Each core can be magnetized in one of two directions, representing binary 0 or 1. This type of memory was commonly used in early computers but has been largely replaced by faster and more efficient technologies. Therefore, core is a magnetic memory.

2. Disk:
A magnetic disk, such as a hard disk drive (HDD), is a non-volatile storage device that uses magnetic fields to store and retrieve digital data. It consists of one or more spinning disks coated with a magnetic material, and data is stored in concentric circles called tracks. Each track is divided into sectors, and data is written and read using read/write heads that move across the disk's surface. Therefore, disk is a magnetic memory.

3. Flip-flop:
A flip-flop is a basic building block of digital circuits and is used for storing a single bit of information. It is not a magnetic memory as it does not utilize magnetization to store or retrieve data. Instead, it is typically implemented using electronic components such as transistors and capacitors. Therefore, flip-flop is not a magnetic memory.

4. Tape:
Magnetic tape is a sequential storage medium that uses a long strip of plastic coated with a magnetic material to store data. It is commonly used for backup and archival purposes due to its large storage capacity and relatively low cost. Data is written and read using a tape drive, which moves the tape past a read/write head. Therefore, tape is a magnetic memory.

Conclusion:
Based on the explanations above, the correct answer is option C: Flip-flop. Flip-flops are not magnetic memories as they do not use magnetization to store or retrieve data.