answer-RMgX

dissociation energy of some R-X are summarized below.we can see the R-F is the strongest. that means it is harder to dissociation compared with R-Cl and R-Br. The same way R-Cl is more stable than R-Br. so in this reaction we will find we will get the R-Br dissociation to form grignard reagent.

dissociation energy→stability→reactivity.  R-Br is less stable than R-Cl. reactivity is better. so it reacts with Mg. understood?

It is a really simple question. There is no exactly rules.just use their stability you can know which one is more likely to react with Mg. Low stablilty. More likely to react. Higher stability more likely not to react.