If I don’t want to study Science at tertiary level and want to select a science for HSC or IBDP, which one should I choose?

Choose whatever interests you. If you select one for a perceived scaling advantage, you might not be motivated to do well if you don’t enjoy it. The critical thinking and problem solving in Science applies to everyday life. The skills that you learn in science will support you in every aspect of your future life – even if, at the moment, you are not sure if you will study Science at tertiary level.