Transition elements are

Options

A. Fe, Co, Nl
B. Li, Na, K
C. Cl, Br, I
D. Ba, Sr, Ca

Explanation

**Step-by-Step Solution:**

1. **Definition of Transition Elements:**
Transition elements are defined as elements that have partially filled d-orbitals. This means that in their electron configuration, the d-subshell is not fully occupied.

2. **Identification of Transition Elements:**
Transition elements are primarily found in the d-block of the periodic table. This block includes groups 3 to 12, where the d-orbitals are being filled with electrons.

3. **Examples of Transition Elements:**
Common examples of transition elements include iron (Fe), cobalt (Co), and nickel (Ni). These elements have partially filled d-orbitals, which is a characteristic feature of transition metals.

4. **Distinction from Other Blocks:**
- S-block elements include groups 1 and 2, such as lithium (Li), sodium (Na), and potassium (K).
- P-block elements include groups 13 to 18, such as chlorine (Cl), bromine (Br), and iodine (I).
- Barium (Ba), strontium (Sr), and calcium (Ca) are also s-block elements.

5. **Conclusion:**
Since transition elements are defined as those with partially filled d-orbitals and the elements mentioned (iron, cobalt, nickel) belong to the d-block, they are classified as transition elements.

**Final Answer:**
Transition elements are those elements in which partially filled d-orbitals are present. Examples include iron, cobalt, and nickel.

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