Because I'm not familiar with which blog you're referencing I can only make conjecture. But let's give it a shot:
There are really only 2 distinct types of products in Magento: simple products, which represent real-world items, carry stock, etc.; and virtual products. Configurable, Bundle, and Group products all represent ways to purchase one or more simple product. At the end of the day it's all about the simple product.
One can assume then that the author inferred either one single item of a specific product. At the end of the day that product is likely to be a simple product because of how product types are architected in Magento.
According to the OP the link referenced is a practice test for the Magento Developer Certification. The question itself looks to be a trick question. They state the correct answer to be a product sold individually.