A. Disadvantages of a multicellular organism: Eeeek! Well I am clearly offended being multicellular my self!
Just kidding....
Well lets think about this. There is a huge range to consider when given the title multicellular. We can go from your basic sponges all the way up to your standard wale. So what would be a disadvantage about being multicellular? I would think that it may have to do with the advent of needing to organize these cells into tissues and working organs. I hardly see this as a disadvantage, but I think a better way of looking at this is asking your self, what do unicellular organisms do/have that I a multicellular organism do not? The best way I can answer this is that because we have adapted more derived traits by utilizing multicellularity that we cannot do really simple things to survive. By this I mean that a protozoan, a single celled animal, can diffuse for gas exchange while we require a respiratory system to, for a lack of a better expression, get air in our body, and we also need a circulatory system to bring the oxygenated blood throughout the body. So to answer part A.... I think that a disadvantage would be that multicellular organism need to have more complex systems while a single celled organism can get away with simpler means for survival.
Alrighty then! Par B...
Levels of organization. This is bit generalized so I won't try to go into too much detail, but if you look at the cellular level, live begins with the cell, then grouped to make tissues, then grouped to make organs, then grouped to make organ systems and.....before you know it your a corporeal being! Some examples could be if you were to just start with a single red blood cell, several of these guys would make up blood and believe it or not blood is actually a tissue! You could then go to describe the tissues that make up the circulatory system, and the organ that controls it (heart!), etc.
And Finally....part B
Well I don't know about you but I am happy to say that I am a fully functioning multicellular organism that has an array of cells that work together to make tissues that can do any thing from keep water out of my body or even produce HCL (ACID!!! Pure Awesomeness!) What is so cool about being multicellular? The amount of complexity that can be found with all of these cells working together. The fact that if one cell dies you are fine, the fact that your cells can be differentiated to do different tasks to keep you alive, the fact that all of these cells work in harmony with each other....its absolutely beautiful. If you have taken anatomy or any science class that discusses the complexity of organs and how they function it is purely phenomenal! So advantages of a multicellular organism? Complex organ systems that work in separate functions like digestion, circulation, respiration, mechanical movement, all derived to help you survive in whatever environment you may live in.