The transition element iron belongs to `ul"4d-series of transition elements."`

Explanation

To determine the correct series of transition elements to which iron belongs, we need to analyze its atomic number and electronic configuration. Here’s a step-by-step solution:

### Step 1: Identify the Atomic Number of Iron
Iron (Fe) has an atomic number of 26. This is a key piece of information that will help us understand its placement in the periodic table.

**Hint:** Remember that the atomic number indicates the number of protons in an atom and helps in identifying the element.

### Step 2: Write the Electronic Configuration of Iron
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