Single Seed Descent (SSD) is a modification of which one of the following methods of selection

Single Seed Descent (SSD) is a modification of the Bulk method of selection.
SSD is the modification of the bulk method. It is first proposed by Goulden (1939) and later modified by Brim (1966).
Instead of bulking a whole seed lot of selected plants, a single seed is selected randomly from each selected plant to make bulk.
The SSD method consists of advancing hybrid populations by taking a single seed from each plant and compositing the seeds to perpetuate the next generation.