Product diversification involves expanding the product range, which has several drawbacks. Risk involvement means entering unknown markets with uncertain outcomes. The trap of full-line competition occurs when competitors attack across all product lines, and huge investment is required for R&D, production, and marketing of new products. Since all options A, B, and C are actual demerits of product diversification, option D correctly identifies that none of these is 'NOT a demerit' - meaning all are demerits.