To solve this problem, we need to analyze each statement about lipids and determine their correctness:$\newline \bullet$(a) Lipids having only single bonds are called unsaturated fatty acids.$\newline$ - This statement is incorrect. Lipids with only single bonds are called saturated fatty acids. Unsaturated fatty acids contain one or more double bonds.$\newline \bullet$(b) Lecithin is a phospholipid.$\newline$ - This statement is correct. Lecithin is indeed a type of phospholipid, commonly found in egg yolks and soybeans.$\newline \bullet$(c) Trihydroxy propane is glycerol.$\newline$ - This statement is correct. Glycerol, also known as glycerin, is a trihydroxy alcohol with the formula $C_3H_8O_3. \newline \bullet$(d) Palmitic acid has 20 carbon atoms including carboxyl carbon.$\newline$ - This statement is incorrect. Palmitic acid is a saturated fatty acid with 16 carbon atoms, including the carboxyl carbon.$\newline \bullet$(e) Arachidonic acid has 16 carbon atoms.$\newline$ - This statement is incorrect. Arachidonic acid is an unsaturated fatty acid with 20 carbon atoms.$\newline$Now, let's identify the correct statements:$\newline$The correct statements are (b) and (c).$\newline$Therefore, the correct answer is Option 3: (b) and (c) only.