Well you should always have tried something, i.e. outline your ideas then people can just help to guide you along. Remember, if you don't do that and just rely on someone explaining from zero, you risk losing it all big time in the real exam. By then it becomes too late.Help please!!!
Go for option 2 => since these involve specific isotopes, they both have mass of 34. Even if you remove some of the dioxygen, the average mass will remain the same.
Check number 1. Same deal with option 2. Average mass will just be 34 because all the molecules present have mass of 34.
Check number 3. H2S + 3/2 O2 --> SO2 + H2O (to balance the eqn - gcse/o level, no). I leave you to think about limiting and excess reactant based on the reacting mole ratio, and the information that you have equal number of these two molecules in the mixture. It all comes down is 3 correct or not.