To solve this problem, we need to determine why the greater part of secondary xylem in old trees is dark brown and resistant to insect attack. Let's analyze each option:$\newline \bullet$(a) Secretion of secondary metabolites and their deposition in the lumen of vessels:$\newline$ - Secondary metabolites such as lignin, tannins, and other phenolic compounds are often deposited in the xylem.$\newline$ - These compounds can make the wood more resistant to decay and insect attack.$\newline$ - This option is relevant to the question.$\newline \bullet$(b) Deposition of organic compounds like tannins and resins in the central layers of stem:$\newline$