Step 1: Understanding the word "cruel."
The word "cruel" means causing pain or suffering, or being unkind. It refers to an individual who is harsh and lacks compassion.
Step 2: Analyzing the options.
- **Merciless** means showing no mercy, which is a synonym for "cruel," not the opposite.
- **Brutal** refers to something harsh or violent, which is similar to "cruel."
- **Unkind** refers to being mean, which is related to cruelty but does not convey the opposite meaning.
- **Kind** refers to being compassionate and considerate, which is the opposite of cruel.
Step 3: Conclusion.
The correct antonym of "cruel" is **kind**, as it refers to being compassionate and gentle.
Final Answer: \[ \boxed{\text{The correct answer is (D) kind.}} \]