If the elements 'A', 'B', 'C' and 'D' are placed in a queue and are deleted one at a time, in what order will these elements be removed?

Question

Accepted Answer

The correct option is (1):
The fundamental principle governing the operation of a Queue data structure is **FIFO**, which stands for **First-In, First-Out**. This concept is directly analogous to a line of people waiting for service, where the person who arrives first is the first to be served. In this question, the elements 'A', 'B', 'C', and 'D' are placed into the queue sequentially, meaning they are enqueued in that specific order. According to the FIFO rule, the element that has been in the queue the longest is the one that must be removed next. Since 'A' was the first element inserted, it is at the front of the line and will be the first element deleted. Following 'A' will be 'B', then 'C', and finally 'D'. Therefore, the order of removal, or *dequeue* operation, must exactly match the order of insertion: ABCD.

The incorrect options are (2), (3), and (4):
The options DCBA, DCAB, and ABDC are incorrect because they all violate the First-In, First-Out (FIFO) principle that is the defining characteristic of a queue. Option (2) **DCBA** represents the exact reverse order of insertion. This Last-In, First-Out (LIFO) behavior is only true for a **Stack** data structure, where 'D' (the last one inserted) would be the first one removed. Options (3) **DCAB** and (4) **ABDC** represent arbitrary, non-sequential deletion orders. A standard queue is restrictive, permitting insertion only at the rear and deletion only from the front. Any result other than the insertion order (ABCD) would imply a violation of the queue's access rules, such as removing elements from the rear or accessing elements randomly from the middle, which is not permitted.