Final answer:
12-crown-4 is the crown ether that solvates lithium ions due to its size that is suitable for coordinating small metal ions such as lithium.
Step-by-step explanation:
The crown ether that solvates lithium ions is 12-crown-4. Crown ethers are cyclic polyethers with a central cavity that can accommodate various metal ions. Due to the size of this cavity, different crown ethers are suitable for coordinating different metal ions. Lithium ions are relatively small compared to other group 1 metal ions and are best coordinated by the smaller 12-crown-4.
Although all the mentioned crown ethers can solvate metal ions, the proper fit is crucial for effective solvation. 18-crown-6 is better suited to solvate larger ions such as potassium because its cavity is larger. The numbers in a crown ether's name, such as "12" in 12-crown-4, indicate the number of atoms that make up the ring, and this largely determines the size of the metal ion they can solvate.