After knowing the statement of Parenthesis Theorem, the solution is straightforward.
Given that discovery time and finish time of the vertices are as follows:
P = [5, 12]
Q= [6, 10]
R= [14, 18]
Here,
the interval of Q is contained entirely within the interval of P
⇒ Q is a descendant of P in a DFS tree (i.e. P and Q belong to the same DFS tree)
Again,
the interval of R and P are entirely disjoint
⇒ Neither R nor P is a descendant of the other
⇒ R and P are in different DFS trees (i.e. R and P belong to different DFS trees)
Hence, option D is correct. (Refer the image attached)
Further, all other similar questions can also be easily solved with this Theorem.