Question 1161127
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By definition, "a tree is an undirected graph in which any two vertices are connected by exactly one path"


More info
<a href = "https://en.wikipedia.org/wiki/Tree_(graph_theory)">https://en.wikipedia.org/wiki/Tree_(graph_theory)</a>


From this definition, we see that graph D fits the description. <font color=red size=4>Therefore you have the correct answer.</font>


In contrast, something like graph A has these two vertices marked in red, along with these two paths marked as well (in blue and green)
<img src = "https://i.imgur.com/xgrCX5k.png">
The fact we have more than one path between those two red vertices shows we do not have a tree for graph A. Similar situations arise with graphs B and C.
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