See, they want to know dijkastra works for linear time(O(N) time, where N is no. of vertex.)
In general dijkastra works in O(E+Nlog N)
Now, we have to think about a data structure which work for dijkastra in O(N) time.
Now, it can be array, stack, queue. Which one suitable?
We have to go each path and in each turn we have to find shortest path. This is nothing but BFS. And BFS works for Queue. So, answer queue here.