The correct answer is option 'C': Cache.

Cache memory acts as a buffer between the CPU (Central Processing Unit) and the main memory. It is a small, high-speed memory that stores frequently accessed data and instructions. The primary purpose of cache memory is to improve the overall performance of the computer system by reducing the average time taken to access data from the main memory.

Here is a detailed explanation of how cache memory works as a buffer between the CPU and main memory:

1. Introduction to Cache Memory:
Cache memory is a small but very fast memory that is located closer to the CPU than the main memory. It stores copies of frequently accessed instructions and data. The cache memory is much faster than the main memory, which helps reduce the CPU's waiting time for data retrieval.

2. Principle of Locality:
Cache memory works based on the principle of locality, which states that programs tend to access a small portion of the available memory at any given time. There are two types of locality: temporal locality and spatial locality.

- Temporal Locality: This refers to the reuse of recently accessed data or instructions. If a particular memory location is accessed, it is likely that it will be accessed again in the near future.
- Spatial Locality: This refers to accessing data or instructions that are physically close to the recently accessed memory location. When the CPU accesses a particular memory location, it is likely to access nearby memory locations as well.

3. Cache Hierarchy:
Modern computer systems typically have multiple levels of cache memory, known as a cache hierarchy. The cache hierarchy consists of several levels of cache, with each level being larger but slower than the previous level. The cache hierarchy helps to bridge the speed gap between the CPU and the main memory.

4. Cache Operation:
When the CPU needs to read data or instructions, it first checks the cache memory. If the required information is present in the cache (cache hit), it can be quickly retrieved, saving time compared to accessing the main memory. If the required information is not present in the cache (cache miss), a slower access to the main memory is required to retrieve the data or instructions. The cache is then updated with the newly accessed data, so that it can be quickly retrieved if needed again in the future.

5. Cache Organization:
Cache memory is organized into cache lines or cache blocks, which are small chunks of data or instructions. When a cache line is loaded, it brings in multiple memory locations that are physically close to the requested location, taking advantage of spatial locality.

In conclusion, cache memory acts as a buffer between the CPU and main memory by storing frequently accessed data and instructions. It helps reduce the average access time and improves the overall performance of the computer system.