Q.What is the main criticism of Marshall's welfare definition of economics?

Marshall's definition improved on Smith's by focusing on human welfare rather than wealth alone, but it still drew a sharp objection.

Marshall defined economics as a study of mankind in the ordinary business of life, concerned with how people earn and use income to promote material welfare. This was an improvement over Smith because it made human welfare, not wealth, the ultimate purpose of economic activity. However, Lionel Robbins pointed out a serious limitation: by tying economics so closely to 'material' welfare, Marshall's definition implicitly excludes many activities that are unmistakably economic but do not obviously promote welfare — for example, the production of tobacco or liquor, or wartime economic activity, or the labour of a non-material service like a singer's performance. Robbins argued that whether or not an activity promotes welfare is irrelevant to whether it is 'economic' — what matters is only whether it involves allocating scarce means among alternative ends. This criticism is what led Robbins to propose the scarcity definition instead.