(D) Letters, numbers and special characters
A strong password is a mix of small and capital letters, numbers, and special characters, making it difficult to guess or hack.
Source: Protecting your Data, Chapter 3
The passage clearly states that passwords are hard to hack when they combine small letters, capital letters, numbers, and special characters. Options (A), (B), and (C) are incomplete — only option (D) covers all elements. Always remember this complete combination for any password-related question.