_________play(s) the most important role in making sure neurons can function effectivly.

A.) The cell body
B.) Dendrites
C.) Synapes
D.) Gila Cells

I think the answer is A. Am I right?

All of the first three play important parts in effective functioning. Which is most important is a matter of opinion. I would pick synapses, because without them, you would be one giant neuron, either firing or not firing.