How are blocks of data “chained” together to ensure the integrity of transactions?

How are blocks of data “chained” together to ensure the integrity of transactions?
A . The hash of the previous block is written to the header of the current block
B . The ID of the previous block is stored in the current block
C . A public key is stored in each block for data decryption
D . The hash of the genesis block is kept in all block headers

Answer: A