B

X-rays are a form of electromagnetic radiation and interact with matter differently depending on the material's density and atomic number.
Step 1: Consider the properties of each material.
- Air has a low atomic number and density, allowing X-rays to pass through relatively easily.
- Wood, being a less dense organic material, also allows substantial penetration by X-rays.
- Water has a higher atomic number than air and wood but is still less dense than metals.
- Iron has a high atomic number and density which means it absorbs and attenuates X-rays more effectively than the other materials.
Step 2: Since X-rays are absorbed more in denser materials with higher atomic numbers, the minimum distance traveled by X-rays will be in the material that stops them the most.
Therefore, the correct answer is B: Iron.