Step 1: Understanding the Concept:
This question tests knowledge of prepositions and fixed idiomatic expressions in English.
Step 2: Detailed Explanation:
The phrase "true to one's word" is a common English idiom. It means that a person is reliable and keeps their promises. The preposition that correctly completes this idiom is "to".
- "true of" is incorrect.
- "true for" is used in different contexts (e.g., "This rule is true for everyone").
- "true at" is incorrect.
Step 3: Final Answer:
The correct idiom requires the preposition "to". Hence, the correct option is (D).