A. Checking the pre and post hash values
B. Replacing the symmetric key often
C. Keeping the private key on a removable media
D. Use longer keys
The correct answer is A.
Integrity is verified using the cryptographic solution of checking the pre and post hash values against each other to see if they are exactly the same. This process is usually performed by employing the binary operator of XOR, where if the two binary hash values being compared are exactly the same, the result of the XOR is 0 (zero), but if they are different then the result is a non-zero value. Replacing the symmetric key often is a good idea, it is the basis of ephemeral keying, but it does not related to integrity. Keeping the private key on a removable media is a secure storage mechanisms, but it does not relate to integrity. Use of longer keys is a good security concept for cryptography, as longer keys make brute force attacks more difficult, but that does not relate to integrity.