The correct answer is C, 'Use parameterized queries,' because this technique prevents the database layer from distinguishing between the code and the data, regardless of the user input. This makes it very effective in preventing injection attacks, as it ensures that user input cannot interfere with the query structure. Option A, 'Client-side input validation,' is not effective by itself because attackers can bypass client-side controls. Option B, 'Encrypting user input,' doesn't prevent injection, as the malicious input would still be executed if decrypted for processing. Option D, 'Employing input length restrictions,' can help to some extent but doesn't address the fundamental problem that the user input may be interpreted as code.