I’m assuming this means mass percent, because that is typical of these types of problems. The substance with the highest mass percent of Br would be the one where the molar mass of Br over the molar mass mass of the substance has the highest value. This would require the other component of the substance to have as small of a molar mass as possible, and by inspection of the periodic table, it is evident that this would be lithium, which has a molar mass of 6.941 amu rather than potassium’s 39.10 or cesium’s 132.91. This means that the correct answer should be C. LiBr.
