Compressing the data before encryption is a technique that can be used to make an encryption scheme more resistant to a known plaintext attack. A known plaintext attack is a type of cryptanalysis where the attacker has access to some pairs of plaintext and ciphertext encrypted with the same key, and tries to recover the key or decrypt other ciphertexts. A known plaintext attack can exploit the statistical properties or patterns of the plaintext or the ciphertext to reduce the search space or guess the key. Compressing the data before encryption can reduce the redundancy and increase the entropy of the plaintext, making it harder for the attacker to find any correlations or similarities between the plaintext and the ciphertext. Compressing the data before encryption can also reduce the size of the plaintext, making it more difficult for the attacker to obtain enough plaintext-ciphertext pairs for a successful attack.

The other options are not techniques that can be used to make an encryption scheme more resistant to a known plaintext attack, but rather techniques that can introduce other security issues or inefficiencies. Hashing the data before encryption is not a useful technique, as hashing is a one-way function that cannot be reversed, and the encrypted hash cannot be decrypted to recover the original data. Hashing the data after encryption is also not a useful technique, as hashing does not add any security to the encryption, and the hash can be easily computed by anyone who has access to the ciphertext. Compressing the data after encryption is not a recommended technique, as compression algorithms usually work better on uncompressed data, and compressing the ciphertext can introduce errors or vulnerabilities that can compromise the encryption.