Which of the following BEST protects against a rainbow table attack?

Rainbow tables are built by precomputing hashes for large sets of likely passwords, typically short or simple ones. Increasing password complexity - using length, mixed case, digits, and symbols - forces any rainbow table targeting that password to grow exponentially, making coverage computationally impractical. This directly shrinks the attack surface that precomputed tables can realistically cover.

Symmetric encryption protects data confidentiality in transit or at rest but has no bearing on the precomputed hash lookup method used in a rainbow table attack against stored credentials.

Cryptographic salting is a server-side storage control that appends random data to a password before hashing - while it adds uniqueness per entry, it is an administrative control applied at storage time, not a user-controlled password property.

Hardened OS configurations reduce a system's overall attack surface but do not prevent an attacker from using precomputed tables against hashes that have already been captured.