Which of the following statement is wrong?

Options

A. Among the following elements :
`K, Mn, Ca, Cs, Fe, Cu, Pb, Os, Y`
The number of transition element is `4`.
B. All the lanthanides and actionides belong to `III B` or the `3rd` group
in the periodic table.
C. The inner transition elements belong to the `f`-block of the periodic table and are shown separately at the bottom of the periodic table.
D. The `d`-block elements have variable valency.

Explanation

To determine which statement is wrong among the given options, we will analyze each statement step by step.

### Step 1: Identify the Elements and Their Blocks
We have the following elements: Ca (Calcium), Cs (Cesium), Fe (Iron), Cu (Copper), Pb (Lead), Os (Osmium), and Y (Yttrium).

- **Ca**: S-block (Group 2)
- **Cs**: S-block (Group 1)
- **Fe**: D-block (Transition metal, Group 8)
- **Cu**: D-block (Transition metal, Group 11)
- **Pb**: P-block (Group 14)
- **Os**: D-block (Transition metal, Group 8)
- **Y**: D-block (Transition metal, Group 3)

### Step 2: Count the Transition Elements
From the above classification, the transition elements (D-block elements) are Fe, Cu, Os, and Y.

- Total D-block elements = 4 (Fe, Cu, Os, Y)

### Step 3: Evaluate the First Statement
The first statement claims that the number of transition elements is four. Since we identified four transition elements, this statement is **correct**.

### Step 4: Evaluate the Second Statement
The second statement claims that all lanthanides and actinides belong to the 3B or third group in the periodic table.

- Lanthanides and actinides are indeed considered inner transition metals and are typically placed in the F-block of the periodic table. However, they are associated with the 3B group in terms of their electron configuration.

This statement is somewhat misleading but can be considered **correct** in the context given.

### Step 5: Evaluate the Third Statement
The third statement claims that inner transition elements belong to the F-block of the periodic table and are shown separately.

- This statement is **correct** as lanthanides and actinides are indeed F-block elements and are displayed separately at the bottom of the periodic table.

### Step 6: Evaluate the Fourth Statement
The fourth statement claims that D-block elements have variable valency.

- This is also **correct**. For example, iron can exhibit valencies of +2 and +3, and mercury can exhibit +1 and +2.

### Conclusion
After evaluating all the statements, the first statement is the only one that is incorrect in the context of the question, as it does not accurately reflect the total number of transition elements when considering the provided list.

### Final Answer
The wrong statement is the first one.

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