Plants in the desert a. effectively conserve and retain water b. have high levels of transpiration c. release 50-80% of water back into the ecosystem d. make their own precipitation

The answer is A and here is why:
We know that plants conserve and retain water. But the reason they do that is so they have enough water to survive. If the plants transpire more, they would lose water, so we know it isn't B. Same thing for C, if they give back 50-80% of the water, they are losing water. And it is obviously impossible for desert plants to make their own precipitation because precipitation is rain and snow and no plant can create that. So, using the process of elimination we can determine that the answer to the question is definitely A. effectively conserve and retain water  

The correct answer is b