B

Step 1: Understand that both wires are made of different materials (rubber and iron) but are identical in length and cross-sectional area.
Step 2: Consider the densities of the materials. Iron has a much higher density (approximately 7.87 g/cm³) compared to rubber (around 1.1 g/cm³). This means that for the same volume, iron will have a greater mass than rubber.
Step 3: Since both wires are stretched by the same weight, the number of atoms is also related to the mass and the molar mass of each material. The number of atoms (n) can be calculated using the formula:

n = \frac{mass}{molar ext{ }mass}.
Step 4: Given that iron is denser and thus has a higher mass for the same volume, its number of moles (and thus atoms) will be lower compared to rubber when both are subjected to the same weight.
Conclusion: Therefore, the number of atoms in the iron wire is less than that of the rubber wire because of the differences in atomic mass and density.
Therefore, the correct answer is: B.