Step 1: Understanding the Concept:
A solution is named by the physical state of the solute and the solvent. We need a solid solute dissolved in a liquid solvent.
Step 2: Check each option:
Hydrogen in palladium: gas solute in solid solvent. Sea water: solid salts (mainly NaCl) dissolved in liquid water, which is solid in liquid. Gels: a liquid dispersed in a solid, a colloid, not a true solution. Amalgam of mercury with metals: liquid (mercury) solute in a solid metal solvent.
Step 3: Conclusion:
Only sea water has a solid solute in a liquid solvent.
Final Answer:
Sea water is a solid-in-liquid solution.
\[ \boxed{\text{(B) }\text{Sea water}} \]