Which of the following is NOT a form of depreciation?

Depreciation refers to the process of allocating the cost of a tangible asset over its useful life, and it is a critical concept in accounting for reflecting the decrease in value of an asset over time. The options provided include various recognized methods of calculating depreciation.

The straight-line method involves the allocation of an equal amount of depreciation expense each year over the asset’s useful life. The reducing balance method accelerates depreciation by applying a fixed percentage to the carrying amount of the asset each year, leading to larger deductions in the earlier years. The sum of the years' digits is another accelerated method where depreciation is calculated by taking the asset's remaining useful life and giving more weight to the earlier years of its life.

However, "reductions cost" does not correspond to any established method of depreciation in accounting practices. It seems to be a misnomer or a non-standard term, thus making it not applicable as a form of depreciation. Therefore, it doesn’t fit into the recognized frameworks used in accounting for asset depreciation.