(a) Association rules are big data when you are doing them on large data sets with many variables. I work at a company that sells tens of thousands of different products and tens of millions of customers. Definitely takes us a while to compute those rules.

(b) The majority of big data is structured. For most big data projects it is typically stored in old school Oracle/Teradata/etc data warehouses and shipped into a Hadoop cluster. It may not be consolidated but it is definitely structured.

(c) The total RAM of our Hadoop cluster is 4TB and ours is small. I would consider that to be big data in the sense that it overwhelms any applications that directly try to access the raw data.