Which of the following is not a variant of merge sort?

Answer: d

Explanation: In-place merge sort, top-down merge sort, and bottom-up merge sort are all recognized variants of merge sort. In-place merge sort reduces space complexity, top-down uses recursion, and bottom-up uses iteration. However, linear merge sort implies O(n) time complexity, which is not possible for comparison-based sorting algorithms. The minimum time complexity for any comparison-based sort is O(n log n), so linear merge sort is not a valid variant.