(a) That's wrong. In this case A has done part of what he promised to do. He has delivered all of the promised tables, but none of the matching chairs.
If there is only partial performance (that is less than substantial performance) of one or more conditions in the contract, or if partial performance gives rise to a sufficiently serious breach of an innominate term, the non-defaulting party may reject that performance and put a stop to any further performance.
In this case, it is obviously a condition of the contract that A deliver 200 matching chairs with the 50 tables. Accordingly, even though there has been partial performance, B is entitled to reject the delivery of the tables and put a stop to any further performance of the contract by A. B would also be entitled to claim damages from A to compensate for any losses flowing from the breach, for example, if it cost him more to get equivalent furniture elsewhere.
It must be noted that partial performance of a warranty, or performance that amounts to less than a serious breach of an innominate term, does not justify termination of performance. If a party who is entitled to reject partial performance chooses to accept it, they must pay pro rata (in proportion) for what they have received.