The wrong statement regarding transition metals among the following is:

Options

A. `4s` electrons penetrate toward the nucleus more than `3d` electrons
B. atomic radii of transition metals increase rapidly with increase in atomic number because of poor shielding of nuclear attraction by `(n-1)` of electrons
C. second and third transition series elements have nearly the same size
D. their densities are higher and densities of the `5d` series element are higher than those of `4d` series elements.

Explanation

To solve the question regarding the wrong statement about transition metals, let's analyze each option step by step.

### Step-by-Step Solution:

1. **Option A: "4s electron penetrates toward nucleus more than 3d electron."**
- The 4s orbital is spherical in shape, which allows it to penetrate closer to the nucleus compared to the 3d orbital, which has a more complex shape (double dumbbell). Therefore, this statement is **correct**.

2. **Option B: "Atomic radius of transition metal increases rapidly with increase in atomic number because of poor shielding of nucleus attraction by n-1 electron."**
- As we move across the transition metals, the atomic number increases, leading to an increase in nuclear charge. However, the increase in effective nuclear charge (Z_eff) results in a stronger attraction between the nucleus and the electrons, causing the atomic radius to **decrease**, not increase. Thus, this statement is **wrong**.

3. **Option C: "2nd and 3rd transition series elements have nearly the same size."**
- The similarity in size between the 2nd and 3rd transition series elements is attributed to the lanthanide contraction. This contraction leads to a smaller size of the 4f orbitals, which affects the size of the 5d elements. Therefore, this statement is **correct**.

4. **Option D: "Their densities are higher and densities of 5D series elements are higher than those of 4D series elements."**
- The densities of 5d series elements are indeed higher than those of the 4d series elements due to the presence of more unpaired electrons, which enhances metallic bonding and increases density. Thus, this statement is **correct**.

### Conclusion:
The wrong statement regarding transition metals is **Option B**.

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