Which of the following does not form double salts?

Step-by-step solution

We know that the double salts are the salts that exist only in the solid phase i.e. crystal lattice, and they cannot retain their identity on dilution in the water but in case of the complex salts their identity retained in solid phase as well as during dilution in the water. Now, here Li is only able to form monovalent ions due to presence of own electrons which is lost in the reaction because lithium is considered one of the most reactive element and as it is smallest among all. Due to all the mentioned reasons L