Final answer:
The term 'vintage' on an Australian wine label indicates the year of harvest, which is crucial for determining the wine's quality and authenticating its age. Option C is correct.
Step-by-step explanation:
The designation of a vintage on an Australian wine label refers to the option C which is year of harvest.
The vintage year on a wine bottle is significant as it indicates the quality of the wine, with some years being better than others due to various climatic conditions. It is also of interest to those who wish to verify the authenticity of a wine's vintage, with techniques such as measuring the radioactivity levels of cesium-137 without opening the bottle.