Explanation:

- Option 1: Diamond

- A diamond is a solid form of carbon and a crystal.

- It's not formed by dissolving anything in a solvent, so not a solution.

- Option 2: Sea Water

- Sea water is a mixture of water and various dissolved salts.

- It fits the definition of a solution where solutes are dissolved in a solvent.

- Sea Water is indeed a solution.

- Option 3: Salt

- Salt is a pure substance and a compound.

- On its own, it's not a solution since it's not dissolved.

- Option 4: Coal

- Coal is a complex mixture of carbon-rich compounds.

- It's not dissolved in a solvent and is not a solution.