The correct answer is Iodine solution.

Iodine solution is used as a chemical test to detect the presence of starch in food substances. When iodine solution, which is typically a brownish-yellow color, is added to a substance containing starch, a distinct color change occurs. The iodine molecules interact with the helical structure of the starch molecules, forming a complex that results in a deep blue-black or purplish-black color. This color change serves as a positive indication for the presence of starch.

Other solutions listed do not yield such a characteristic color change with starch. Copper sulphate solution is used in different chemical tests, often related to identifying reducing sugars. Ammonium chloride and chlorine solution have different applications and are not associated with starch detection.

The test is simple, rapid, and provides a clear visual indication, making iodine solution the preferred reagent for starch identification in both qualitative and sometimes quantitative analyses in food science and related fields. The intensity of the color change can even provide an indication of the concentration of starch present, though more precise methods are employed for quantitative measurement.