What is an external sorting algorithm?

There are two types of algorithms with respect to space complexity. They are in-place and out-of-place algorithms.

In-place algorithms: These algorithm do not require extra memory allocation while performing sorting, searching or deletion operations on a particular set of data in a data structure.

out-of-place algorithms: This algorithm is also known as external sorting algorithm. These algorithms require extra space sometime equal to the number of inputs in the original data structure. For very large dataset, computer memory does not fulfill the requirements due to which external tapes or disks have to be used during the operations like sorting.

Hence, the correct option is A i.e., Algorithm that uses tape or disk during the sort.