Lanthanide contraction is observed in:

Options

A. Gd
B. At
C. Xe
D. Ac

Answer

### Step-by-Step Solution:

1. **Understanding Lanthanide Contraction**:
- Lanthanide contraction refers to the gradual decrease in the size of the atoms and ions of the lanthanide series elements as you move from lanthanum (La, atomic number 57) to lutetium (Lu, atomic number 71). This contraction occurs due to the poor shielding effect of the f-electrons, which leads to an increase in effective nuclear charge.

2. **Identifying the Lanthanide Series**:
- The lanthanide series includes the elements from atomic number 57 (Lanthanum) to atomic number 71 (Lutetium). The elements in this series are:
- La (57), Ce (58), Pr (59), Nd (60), Pm (61), Sm (62), Eu (63), Gd (64), Tb (65), Dy (66), Ho (67), Er (68), Tm (69), Yb (70), Lu (71).

3. **Analyzing the Options**:
- The question provides four options: Gd (Gadolinium, atomic number 64), Ac (Actinium, atomic number 89), Xe (Xenon, atomic number 54), and At (Astatine, atomic number 85).
- Among these, only Gd falls within the lanthanide series.

4. **Eliminating Incorrect Options**:
- **Ac (Actinium)**: This element is part of the actinide series (atomic number 89) and does not exhibit lanthanide contraction.
- **Xe (Xenon)**: This is an inert gas (atomic number 54) and is not part of the lanthanide series.
- **At (Astatine)**: This is a radioactive halogen (atomic number 85) and also does not belong to the lanthanide series.

5. **Conclusion**:
- The only element that exhibits lanthanide contraction from the provided options is Gd (Gadolinium, atomic number 64).

### Final Answer:
- **Gd (Gadolinium)** is the correct answer as it is the only element from the lanthanide series listed in the options.