Answer is (1) - Rb.
To form MCl, The cation should have +1 as the charge. According to the given choices, Rb can form +1 ion by losing an electron. It is because Rb is in 1st group of the periodic table.
The electron configuration of Rb is [Kr] 5s¹. After losing as electron, Rb has electron configuration as [Kr].
The compound is RbCl (Rubidium Chloride) which has a molar mass of 120.921 g/mol.