A sorting technique is called stable if
it maintains the relative order of occurrence of non-distinct elements
it uses divide and conquer paradigm
it takes $O(n)$ space
(B) If it maintains the relative order of occurrence of non-distinct elements.
(from definition of stable sorting)
A sorting algorithm is said to be stable if two objects with equal keys appear in the same order in sorted output as they appear in the input array to be sorted. Some sorting algorithms are stable by nature like Insertion sort, Merge Sort, Bubble Sort, etc. And some sorting algorithms are not, like Heap Sort, Quick Sort, etc.