Which of these is likely to result in the most competition? a. organisms that live in different ecosystems b. organisms that use the same resource c. organisms that live in the same area d. organisms that are classified in the same phylum

It wouldn't be A because the animals wouldn't really be bothered by each other in the first place. They're not in the same ecosystems..
It wouldn't be D because these animals would be ranked the same if they were in the same phylum.
It could be C, but it's not because there is a variety of other resources for them to use. 
I think B would be the best answer because they are fighting each other for the same resource. You need that resource in order to survive and so does that other organism, so of course both organisms would be fighting over it in order to live

hope this helps!

B because they two organisms are competing over a common resource

Well let's uhmmm how many bedrooms you want. if you want a garage. not to mention the price of the house?