In which of the following pairs, the atomic size is almost the same ?

Options

A. La - Ce
B. Nb - Tb
C. Zr - Hf
D. Nb - Zr

Explanation

To determine which pair of elements has almost the same atomic size, we need to analyze the atomic sizes of the given pairs. The question provides us with four options, and we will evaluate each pair based on their atomic numbers and sizes.

### Step-by-Step Solution:

1. **Identify the pairs and their atomic numbers:**
- **Option A:** Lanthanum (La, Atomic number 57) and Cerium (Ce, Atomic number 58)
- **Option B:** Niobium (Nb, Atomic number 41) and Terbium (Tb, Atomic number 65)
- **Option C:** Zirconium (Zr, Atomic number 40) and Hafnium (Hf, Atomic number 72)
- **Option D:** Niobium (Nb, Atomic number 41) and Zirconium (Zr, Atomic number 40)

2. **Look up the atomic sizes:**
- **Lanthanum (La):** 250 pm
- **Cerium (Ce):** 233 pm
- **Niobium (Nb):** 198 pm
- **Terbium (Tb):** 207 pm
- **Zirconium (Zr):** 206 pm
- **Hafnium (Hf):** 208 pm

3. **Calculate the differences in atomic sizes for each pair:**
- **Option A (La and Ce):**
- Difference = |250 pm - 233 pm| = 17 pm (not almost the same)
- **Option B (Nb and Tb):**
- Difference = |198 pm - 207 pm| = 9 pm (not almost the same)
- **Option C (Zr and Hf):**
- Difference = |206 pm - 208 pm| = 2 pm (almost the same)
- **Option D (Nb and Zr):**
- Difference = |198 pm - 206 pm| = 8 pm (not almost the same)

4. **Conclusion:**
- The pair with atomic sizes that are almost the same is **Option C: Zirconium (Zr) and Hafnium (Hf)**, with sizes of 206 pm and 208 pm respectively.

### Final Answer:
The pair in which the atomic size is almost the same is **C: Zirconium (Zr) and Hafnium (Hf)**.