Five-kingdom classification was proposed by __________ .

Explanation

**Step-by-Step Solution:**

1. **Understand the Question**: The question asks who proposed the five-kingdom classification system.

2. **Identify Key Information**: The five-kingdom classification is a system that categorizes all living organisms into five distinct groups.

3. **Recall the Contributor**: The five-kingdom classification system was proposed by R.H. Whittaker.

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