The reactions taking place in one step is called ............... reactions.

Explanation

**Step-by-Step Solution:**

1. **Understanding the Question**: The question asks for the term used to describe reactions that occur in a single step.

2. **Identifying the Term**: In chemistry, reactions that occur in a single step are referred to as "elementary reactions."

3. **Distinguishing from Other Types**: It's important to note that reactions can also be classified as complex reactions, which involve multiple steps.

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