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

1. enqueue (Insertion at head):

   • A new node is allocated, its next pointer is set to the current head, and head is updated to point to the new node.

   • This requires a constant number of operations, taking $\Theta(1)$ time.

2. dequeue (Deletion from tail):

   • To delete the tail node in a singly linked list, we must update the next pointer of the node preceding the tail to NULL (or ground) and point tail to this predecessor.

   • Since the list is singly linked with pointers directed from head towards tail, there are no backward pointers from tail.

   • Finding the predecessor of the tail node requires traversing all $n$ nodes from the head.

   • Hence, deletion at the tail takes $\Theta(n)$ time.

Therefore, the time complexities are $\Theta(1)$ and $\Theta(n)$, making (B) the correct option.