Definition of Back edge :- If v is an ancestor of u, then edge (u, v) is a back edge.
ref :- https://courses.csail.mit.edu/6.006/fall11/rec/rec14.pdf
Now, starting from point p next unvisited nodes {q,r,s} choosing based on alphabetical order. i.e. q
standing at node q next unvisited nodes { r, t} choosing based on alphabetical order. i.e. r
standing at node r next unvisited nodes { v} choosing based on alphabetical order. i.e. v
standing at node v next unvisited nodes { s, t} choosing based on alphabetical order. i.e. s
standing at node s next unvisited nodes { t} choosing based on alphabetical order. i.e. t
All nodes are traversed,
Back edges are as follow :- (t,v), (s,p) , (t,q) (r,p)
Number of back edges = 4.
Hence 4 is the answer.