The answer is A.) Antibodies are formed that fight those types of bacteria.
To explain, your immune system protects you from invasive bacteria by attacking and destroying them. The problem is that your system needs to recognize invading organisms like bacteria. Remember that bacteria cells have many of the same types of structures as your own cells, and you do not want your immune system to attack your own body. Special cells called T-cells and B-cells participate in this process. To make a long story short, antibodies are special molecules that your immune system "labels" invading bacteria with, so that it knows what to attack. These antibodies only fit on molecular structures of the targeted bacteria.
When a dead or weakened bacteria, or even pieces of a bad bacteria are introduced to the body, the B-cells and T-cells create antibodies that label that specific type of bacteria, or even its parts. They also remember how to make them in the future, so that later, when and if infected with a live version of that bacteria, the immune system already makes the antibodies to fight that infection. The process can work for viruses too.
