Step 1: Understanding the Question:
This question asks for the classification of a solution where hydrogen gas ($H_2$) is absorbed by solid palladium (Pd) metal. Step 2: Detailed Explanation:
1. A solution is composed of a solute (the component present in smaller quantity) and a solvent (the component present in larger quantity and which determines the physical state of the solution).
2. In the case of hydrogen in palladium, hydrogen gas molecules occupy the interstitial spaces within the crystal lattice of the solid palladium.
3. Here, the gas ($H_2$) is the solute and the solid (Palladium) is the solvent. Step 4: Final Answer:
The solution of $H_2$ in palladium is a solid solution where gas is the solute and solid is the solvent.