The correct answer is: A. The memory of the data node.
HDFS is a distributed file system that stores data on commodity hardware. It is designed to be scalable, reliable, and fault-tolerant. HDFS divides data into blocks, which are then stored on multiple data nodes. The namenode is responsible for managing the metadata of the file system, such as the location of the blocks.
When a client requests a file from HDFS, the namenode will return the location of the blocks that make up the file. The client will then read the blocks from the data nodes. If a block is frequently accessed, it will be cached in the memory of the data node. This will improve the performance of the file system, as the client will not have to read the block from the disk.
Options B and C are incorrect because the namenode does not store the data blocks. The namenode only stores the metadata of the file system.