(a) That's wrong. The contract is for the sale of goods and is therefore regulated by the sale of goods legislation of the relevant state or territory, such as the Goods Act 1958 (Vic).
In the example, the question is whether the seller is bound to deliver the particular bags of rice referred to at the time of the sale, or whether exactly equivalent goods will do.
The relevant state and territory sale of goods legislation makes it an implied condition of a contract of sale that, when goods are identified by description, the goods delivered must be those that were identified in the description. See a list of the relevant sections.
In this case, A has failed to deliver 'these 10 bags of rice' - he has delivered 10 different bags, which is a breach of the implied term.