<span>When you have to balance an equation it means that you have to balance both parts of equation so that the numbers of each elements will have the same amount. According to this rule, it is quite easy to distinguish unbalanced reaction, and the answer is a) HBr + Ca(OH)2 → CaBr2 + H2O as you can see that amount of oxygen on both sides is not the same.</span>
We see from the reaction side of the equation that we have 2 Li. That means we need the same number of Li on the products side. Therefore, our only option is 2.