A queue is implemented using a non-circular singly linked list. The queue has…

A queue is implemented using a non-circular singly linked list. The queue has a head pointer and a tail pointer, as shown in the figure. Let $n$ denote the number of nodes in the queue. Let $enqueue$ be implemented by inserting a new node at the head, and $dequeue$ be implemented by deletion of a node from the tail.

Which one of the following is the time complexity of the most time-efficient implementation of $enqueue$ and $dequeue$, respectively, for this data structure?